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Research article Special Issues

Duality properties of metric Sobolev spaces and capacity

  • We study the properties of the dual Sobolev space H1,q(X)=(H1,p(X)) on a complete extended metric-topological measure space X=(X,τ,d,m) for p(1,). We will show that a crucial role is played by the strong closure H1,qpd(X) of Lq(X,m) in the dual H1,q(X), which can be identified with the predual of H1,p(X). We will show that positive functionals in H1,q(X) can be represented as a positive Radon measure and we will charaterize their dual norm in terms of a suitable energy functional on nonparametric dynamic plans. As a byproduct, we will show that for every Radon measure μ with finite dual Sobolev energy, Capp-negligible sets are also μ-negligible and good representatives of Sobolev functions belong to L1(X,μ). We eventually show that the Newtonian-Sobolev capacity Capp admits a natural dual representation in terms of such a class of Radon measures.

    Citation: Luigi Ambrosio, Giuseppe Savaré. Duality properties of metric Sobolev spaces and capacity[J]. Mathematics in Engineering, 2021, 3(1): 1-31. doi: 10.3934/mine.2021001

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  • We study the properties of the dual Sobolev space H1,q(X)=(H1,p(X)) on a complete extended metric-topological measure space X=(X,τ,d,m) for p(1,). We will show that a crucial role is played by the strong closure H1,qpd(X) of Lq(X,m) in the dual H1,q(X), which can be identified with the predual of H1,p(X). We will show that positive functionals in H1,q(X) can be represented as a positive Radon measure and we will charaterize their dual norm in terms of a suitable energy functional on nonparametric dynamic plans. As a byproduct, we will show that for every Radon measure μ with finite dual Sobolev energy, Capp-negligible sets are also μ-negligible and good representatives of Sobolev functions belong to L1(X,μ). We eventually show that the Newtonian-Sobolev capacity Capp admits a natural dual representation in terms of such a class of Radon measures.


    Dedicated to Sandro Salsa on the occasion of his 70th birthday.

    In this paper we investigate the properties of the duals of the metric Sobolev spaces H1,p(X), where X=(X,τ,d,m) is an extended metric-topological measure space and p(1,+).

    In the simpler case when (X,d) is a complete and separable metric space, τ is the topology induced by the metric and m is a positive and finite Borel (thus Radon) measure on X, H1,p(X) can be defined as the natural domain of the Lp(X,m)-relaxation of the pre-Cheeger energy form

    pCEp(f):=X(lipf(x))pdm(x),fLipb(X),

    initially defined only for bounded Lipschitz functions. Here lipf(x) defines the asymptotic Lipschitz constant

    lipf(x)=lim supy,zxyz|f(y)f(z)|d(y,z).

    For every function fH1,p(X) one can define the Cheeger energy

    CEp(f):={lim infnpCEp(fn):fnLipb(X), fnf strongly in Lp(X,m)}

    and the Sobolev norm

    fH1,p(X):=(fpLp+CEp(f))1/p,

    thus obtaining a Banach space. It is therefore quite natural to study its dual, which we will denote by H1,q(X).

    In such a general situation, however, when we do not assume any doubling and/or Poincaré assupmptions, H1,p(X) may fail to be reflexive or separable and it is not known if the generating class of bounded Lipschitz functions is strongly dense.

    As a first contribution, we will show that it could be more convenient to consider the smaller subspace H1,qpd(X) of H1,q(X) obtained by taking the strong closure of Lq(X,m). Linear functionals in H1,qpd(X) are characterized by their behaviour on Lipb(X) (or on even smaller generating subalgebras) and their dual norm can also be computed by the formula

    LH1,q(X)=sup{L,f:fLipb(X), pCEp(f)+fpLp1}, (1.1)

    which is well adapted to be applied to general Borel measures μ on X.

    In Sections 3 and 4 we will show that H1,qpd(X) has three important properties:

    (a) it can be identified with the predual of H1,p(X) (thus showing in particular that H1,p(X) is the dual of a separable Banach space);

    (b) every positive Borel measure μ satisfying

    |Xfdμ|C(pCEp(f)+fpLp)1/pfor every fLipb(X)

    can be extended in a unique way to a functional LμH1,qpd(X);

    (c) every positive functional on Lipb(X) such that the supremum in (1.1) is finite can be represented by a positive Radon measure.

    This last property relies on a representation formula of the dual of the Cheeger energy by (nonparametric) dynamic plans (Theorem 4.6) which is interesting by itself. As a further important application of this result, in the final section 5 we will show that negligible sets in E with respect to the Newtonian capacity Capp are also μ-negligible for every positive Borel measure with finite dual energy. As a byproduct, we can express the duality of μ with a function f in H1,p(X) in the integral form

    μ,f=X˜fdμ,

    where ˜f is any good representative of f in the Newtonian space N1,p(X).

    Our last application concerns the variational representation of the Newtonian capacity of a closed set FX

    (Capp(F))1/p=sup{μ(F):μM+(X), μ(XF)=0, LμH1,q(X)1}.

    Main notation
    (X,τ) Hausdorff topological space
    (X,τ,d) Extended metric-topological (e.m.t.) space, see §2.2 and Definition 2.2
    X=(X,τ,d,m) Extended metric-topological measure (e.m.t.m.) space, see §2.2
    M+(X) Positive and finite Radon measures on a Hausdorff topological space X, §2.1
    B(X) Borel subsets of X
    fμ Push forward of μM(X) by a (Lusin μ-measurable) map f:XY, (2.1)
    Cb(X,τ), Cb(X) τ-continuous and bounded real functions on X
    Lipb(X,τ,d) Bounded, τ-continuous and d-Lipschitz real functions on X, (2.2)
    lipf(x) Asymptotic Lipschitz constant of f at a point x, (2.4)
    A Compatible unital sub-algebra of Lipb(X,τ,d), Definition 2.3
    Lq(σ|μ) q-Entropy functionals on Radon measures, (4.2)
    C([0,1];(X,τ)) τ-continuous curves defined in [0,1] with values in X, §2.4
    τC, dC Compact open topology and extended distance on C([0,1];X), §2.4
    BVC([0,1];X) Continuous curves with d-bounded variation, §2.4
    RA(X) Continuous and rectifiable arcs, §2.4
    e(,t),et(),e[] Evaluation maps along curves and arcs, §2.4
    τA, dA Quotient topology and extended distance on RA(X), §2.4
    Rγ Arc-length reparametrization of a rectifiable arc γ, §2.4
    γf Integral of a function f along a rectifiable curve (or arc) γ, §2.4
    (γ) length of γ, §2.4
    νγ Radon measure in M+(X) induced by integration along a rectifiable arc γ, (2.9)
    pCEp, CEp, CEp,κ (pre)Cheeger energy, Definition 3.1
    H1,p(X) Metric Sobolev space induced by the Cheeger energy, Definition 3.1
    |Df| Minimal p-relaxed gradient, (3.1)-(3.2)
    \operatorname{Bar}_{q}( {\boldsymbol \pi}) q -barycentric entropy of a dynamic plan, Definition 4.2
    {{\mathcal{B}}_q}\left( {{\rm{RA}}\left( X \right)} \right) Plans with barycenter in L^q(X, \mathfrak m) , Definition 4.2
    \mathscr{D}_q(\mu_0,\mu_1) Dual dynamic cost, (4.5)
    \operatorname{Mod}_{p}(\Gamma) p -Modulus of a collection \Gamma\subset \mathrm{RA}(X) , Definition 5.1
    N^{1,p}( \mathbb{X}) Newtonian space, Definition 5.3
    \operatorname{Cap}_p Newtonian capacity, (5.2)

    Let (Y,\tau_Y) be a Hausdorff topological space. We will denote by {\mathrm C}_b(Y,\tau_Y) the space of \tau_Y -continuous and bounded real functions defined on Y ; \mathscr{B}(Y,\tau_Y) is the collection of the Borel subsets of Y ; we will often omit the explicit indication of the topology \tau_Y , when it will be clear from the context.

    Definition 2.1 (Radon measures [22, Chap. Ⅰ, Sect. 2]). A finite Radon measure \mu: \mathscr{B}(Y,\tau_Y)\to[0,+\infty) is a Borel nonnegative \sigma -additive finite measure satisfying the following inner regularity property:

    \begin{equation*} \forall\,B\in \mathscr{B}(Y,\tau_Y):\quad \mu(B) = \sup\Big\{\mu(K):K\subset B,\ {K\; compact}\Big\}. \end{equation*}

    We will denote by \mathcal M_+(Y) the collection of all the finite positive Radon measures on Y .

    It is worth mentioning that every Borel measure in a Polish, Lusin, Souslin, or locally compact space with a countable base of open sets is Radon [22, Ch. Ⅱ, Sect. 3]. In particular the notation of \mathcal M_+(Y) is consistent with the standard one adopted e.g., in [4,6,24], where Polish or second countable locally compact spaces are considered.

    If (Y,\tau_Y) is completely regular, the weak (or narrow) topology \tau_{ \mathcal M_+} on \mathcal M_+(Y) can be defined as the coarsest topology for which all maps

    \begin{equation*} \mu\mapsto \int_Y h\, {\mathrm d}\mu\qquad\text{from $ \mathcal M_+(Y)$ into $ \mathbb{R}$} \end{equation*}

    are continuous as h:Y\to \mathbb{R} varies in {\mathrm C}_b(Y,\tau_Y) [22, p. 370,371].

    Recall that a set A\subset Y is \mu -measurable, \mu\in \mathcal M_+(Y) , if there exist Borel sets B_1,B_2\in \mathscr{B}({Y,\tau_Y}) such that B_1\subset A\subset B_2 and \mu(B_2\setminus B_1) = 0 . A set is called universally (Radon) measurable if it is \mu -measurable for every Radon measure \mu\in \mathcal M_+(Y) .

    Let (Z,\tau_Z) be another Hausdorff topological space. A map f:Y\to Z is Borel (resp. \mu -measurable) if for every B\in \mathscr{B}(Z) f^{-1}(B)\in \mathscr{B}(Y) (resp. f^{-1}(B) is \mu -measurable). f is Lusin \mu -measurable if for every \varepsilon>0 there exists a compact set K_ \varepsilon\subset Y such that \mu(Y\setminus K_ \varepsilon)\le \varepsilon and the restriction of f to K_ \varepsilon is continuous. A map f:Y\to Z is called universally Lusin measurable if it is Lusin \mu -measurable for every Radon measure \mu\in \mathcal M_+(Y) .

    Every Lusin \mu -measurable map is also \mu -measurable. Whenever f is Lusin \mu -measurable, its push-forward

    \begin{equation} f_\sharp\mu\in \mathcal M_+(Z),\quad f_\sharp\mu(B): = \mu(f^{-1}(B))\quad \text{for every Borel subset }B\subset \mathscr{B}(Z) \end{equation} (2.1)

    induces a Radon measure in Z .

    Given a power p\in (1,\infty) and a Radon measure \mu in (Y,\tau_Y) we will denote by L^p(Y,\mu) the usual Lebesgue space of class of p -summable \mu -measurable functions defined up to \mu -negligible sets.

    Let (X,\tau) be a Hausdorff topological space. An extended distance is a symmetric map {\mathsf d}:X\times X\to[0,\infty] satisfying the triangle inequality and the property {\mathsf d}(x,y) = 0 iff x = y in X : we call (X, {\mathsf d}) an extended metric space. We will omit the adjective "extended" if {\mathsf d} takes real values.

    Let {\mathsf d} be an extended distance on X . For every f:X\to \mathbb{R} and A\subset X we set

    \begin{equation*} \mathop{{\rm{Lip}}}\nolimits(f,A): = \inf\Big\{L\in [0,\infty]:|f(y)-f(z)|\le L {\mathsf d}(y,z)\quad \text{for every }y,z\in A\Big\}. \end{equation*}

    We adopt the convention to omit the set A when A = X . We consider the class of \tau -continuous and {\mathsf d} -Lipschitz functions

    \begin{equation} \begin{aligned} \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}): = {}&\Big\{f\in {\mathrm C}_b(X,\tau): \mathop{{\rm{Lip}}}\nolimits(f) \lt \infty\Big\}, \end{aligned} \end{equation} (2.2)

    and for every \kappa>0 we will also consider the subsets

    \begin{equation*} \begin{aligned} \mathop{\rm Lip}_{b,\kappa}\nolimits(X,\tau, {\mathsf d}): = {}& \Big\{f\in {\mathrm C}_b(X,\tau): \mathop{{\rm{Lip}}}\nolimits(f)\le \kappa\Big\}. \end{aligned} \end{equation*}

    A particular role will be played by \mathop{\rm Lip}_{b,1}\nolimits(X,\tau, {\mathsf d}) . It is easy to check that \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}) is a real and commutative sub-algebras of {\mathrm C}_b(X,\tau) with unit. According to [2, Definition 4.1], an extended metric-topological space (e.m.t. space) (X,\tau, {\mathsf d}) is characterized by a Hausdorff topology \tau and an extended distance {\mathsf d} satisfying a suitable compatibility condition.

    Definition 2.2 (Extended metric-topological spaces). Let (X, {\mathsf d}) be an extended metric space, let \tau be a Hausdorff topology in X . We say that (X,\tau, {\mathsf d}) is an extended metric-topological (e.m.t.) space if:

    (X1) the topology \tau is generated by the family of functions \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}) ;

    (X2) the distance {\mathsf d} can be recovered by the functions in \mathop{\rm Lip}_{b,1}\nolimits(X,\tau, {\mathsf d}) through the formula

    \begin{equation} {\mathsf d}(x,y) = \sup\limits_{f\in \mathop{\rm Lip}_{b,1}\nolimits(X,\tau, {\mathsf d})}|f(x)-f(y)| \quad{for\; every }\;x,y\in X. \end{equation} (2.3)

    We will say that (X,\tau, {\mathsf d}) is complete if {\mathsf d} -Cauchy sequences are {\mathsf d} -convergent. All the other topological properties usually refer to (X,\tau) .

    The previous assumptions guarantee that (X,\tau) is completely regular. When an e.m.t. space (X,\tau, {\mathsf d}) is provided by a positive Radon measure \mathfrak m\in \mathcal M_+(X,\tau) we will say that

    \begin{equation*} \text{the system $ \mathbb{X} = (X,\tau, {\mathsf d}, \mathfrak m)$ is an extended metric-topological measure (e.m.t.m.) space.} \end{equation*}

    Definition 2.2 yields two important properties linking {\mathsf d} and \tau : first of all

    \begin{equation*} {\mathsf d}\text{ is $\tau\times\tau$-lower semicontinuous in $X\times X$}, \end{equation*}

    since it is the supremum of a family of continuous maps by (2.3). On the other hand, every {\mathsf d} -converging net (x_j)_{j\in J} indexed by a directed set J is also \tau -convergent:

    \begin{equation*} \lim\limits_{j\in J} {\mathsf d}(x_j,x) = 0\quad\Rightarrow\quad \lim\limits_{j\in J}x_j = x\quad\text{w.r.t. $\tau$.} \end{equation*}

    It is sufficient to observe that \tau is the initial topology generated by \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}) so that a net (x_j) is convergent to a point x if and only if

    \begin{equation*} \lim\limits_{j\in J}f(x_j) = f(x)\quad\text{for every }f\in \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}). \end{equation*}

    In many situations it could be useful to consider smaller subalgebras which are however sufficiently rich to recover the metric properties of an extended metric topological space (X,\tau, {\mathsf d}) .

    Definition 2.3 (Compatible algebras of Lipschitz functions). Let \mathscr{A} be a unital subalgebra of \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}) and let us set \mathscr{A}_\kappa: = \mathscr{A}\cap \mathop{\rm Lip}_{b,\kappa}\nolimits(X,\tau, {\mathsf d}) .

    We say that \mathscr{A} is compatible with the metric-topological structure (X,\tau, {\mathsf d}) if

    {\mathsf d}(x,y) = \sup\limits_{f\in \mathscr{A}_1}|f(x)-f(y)|\quad {for\; every }\;x,y\in X.

    In particular, \mathscr{A} separates the points of X .

    It is not difficult to show that any compatible algebra \mathscr{A} is dense in L^p(X, \mathfrak m) [21, Lemma 2.27]. If we do not make a different explicit choice, we will always assume that an e.m.t.m. space \mathbb{X} is endowed with the canonical algebra \mathscr{A}( \mathbb{X}): = \mathop{{\rm{Lip}}}_b\nolimits(X,\tau, {\mathsf d}) .

    For every f:X\to \mathbb{R} and x\in X , denoting by \mathscr{U}_x the directed set of all the \tau -neighborhoods of x , we set

    \mathop{\rm{ lip }}\nolimits f(x): = \lim\limits_{U\in \mathscr{U}_x} \mathop{\rm{ Lip }}\nolimits(f,U) = \inf\limits_{U\in \mathscr{U}_x} \mathop{\rm{ Lip }}\nolimits(f,U)\quad x\in X.

    Notice that \mathop{{\rm{Lip}}}\nolimits(f,\{x\}) = 0 and therefore \mathop{{\rm{lip}}}\nolimits f(x) = 0 if x is an isolated point of X . We can also define \mathop{{\rm{lip}}}\nolimits f as

    \begin{equation} \mathop{{\rm{lip}}}\nolimits f(x) = \limsup\limits_{y,z\to x\atop y\neq z}\frac{|f(y)-f(z)|}{ {\mathsf d}(y,z)}, \end{equation} (2.4)

    where the convergence of y,z to x in (2.4) is intended with respect to the topology \tau . In particular,

    \begin{equation} \mathop{{\rm{lip}}}\nolimits f(x)\ge | {\mathrm D} f|(x): = \limsup\limits_{y\to x}\frac{|f(y)-f(x)|}{ {\mathsf d}(x,y)}. \end{equation} (2.5)

    It is not difficult to check that x\mapsto \mathop{{\rm{lip}}}\nolimits f(x) is a \tau -upper semicontinuous map and f is locally {\mathsf d} -Lipschitz in X iff \mathop{{\rm{lip}}}\nolimits f(x)<\infty for every x\in X . When (X, {\mathsf d}) is a length space, \mathop{{\rm{lip}}}\nolimits f coincides with the upper \tau -semicontinuous envelope of the local Lipschitz constant (2.5).

    We collect in the next useful lemma the basic calculus properties of \mathop{{\rm{lip}}}\nolimits f .

    Lemma 2.4. For every f,g, {\raise.3ex\hbox{ $\chi$ }}\in {\mathrm C}_b(X) with {\raise.3ex\hbox{ $\chi$ }}(X)\subset [0,1] we have

    \begin{align*} \mathop{{\rm{lip}}}\nolimits (\alpha f+\beta g)&\le |\alpha|\, \mathop{{\rm{lip}}}\nolimits f+|\beta|\, \mathop{{\rm{lip}}}\nolimits g\quad\mathit{\text{for every}}\;\alpha,\beta\in \mathbb{R},\\ \mathop{{\rm{lip}}}\nolimits(fg)&\le |f| \mathop{{\rm{lip}}}\nolimits g+|g| \mathop{{\rm{lip}}}\nolimits f,\\ \mathop{{\rm{lip}}}\nolimits((1- {\raise.3ex\hbox{$\chi$}})f+ {\raise.3ex\hbox{$\chi$}} g)&\le (1- {\raise.3ex\hbox{$\chi$}}) \mathop{{\rm{lip}}}\nolimits f+ {\raise.3ex\hbox{$\chi$}} \mathop{{\rm{lip}}}\nolimits g+ \mathop{{\rm{lip}}}\nolimits {\raise.3ex\hbox{$\chi$}}|f-g|. \end{align*}

    Moreover, whenever \phi\in {\mathrm C}^1( \mathbb{R})

    \mathop{{\rm{lip}}}\nolimits(\phi\circ f)= |\phi'\circ f| \mathop{{\rm{lip}}}\nolimits f\,.

    We briefly recap some useful results concerning the extended metric-topological structure of the space of rectifiable arcs in an e.m.t. space (X,\tau, {\mathsf d}) . We refer to [21, §3] for a more detailed discussion and for the related proofs.

    For every \gamma:[0,1]\to X and t\in [0,1] we set

    V_\gamma(t): = \sup\Big\{\sum\limits_{j = 1}^N {\mathsf d}(\gamma(t_j),\gamma(t_{j-1})): 0 = t_0 \lt t_1 \lt \cdots \lt t_N = t\ \Big\}, \quad \ell(\gamma): = V_\gamma(1).

    \mathrm{BVC}([0,1];X) will denote the space of {\mathsf d} -continuous maps \gamma:[0,1]\to X such that \ell(\gamma)<\infty ; notice that if \ell(\gamma) = 0 then \gamma is constant. We will also consider the set of curves with constant velocity

    \begin{equation} \mathrm{BVC}_c([0,1];X): = \big\{\gamma\in \mathrm{BVC}([0,1];X): V_\gamma(t) = \ell(\gamma)t\big\}. \end{equation} (2.6)

    Notice that for every \gamma\in \mathrm{BVC}([0,1];X) the map V_\gamma:[0,1]\to [0, \ell(\gamma)] is continuous and surjective and whenever \ell(\gamma)>0

    \begin{equation} \begin{gathered} \text{there exists a unique $ \ell(\gamma)$-Lipschitz map $ R_\gamma\in \mathrm{BVC}_c([0,1];X)$ such that}\\ \gamma(t) = R_\gamma( \ell(\gamma)^{-1}V_{\gamma}(t))\quad\text{for every $t\in [0,1]$}, \end{gathered} \end{equation} (2.7)

    with | R_\gamma'|(s) = \ell(\gamma) a.e.; when \ell(\gamma) = 0 then R_\gamma(t) = \gamma(t) is constant. We can use R_\gamma to define the integral of a bounded or nonnegative Borel function f:X\to \mathbb{R} along \gamma :

    \begin{equation} \int_\gamma f = \int_0^{1} f( R_\gamma(s)) | R_\gamma'|(s)\, {\mathrm d} s = \ell(\gamma)\int_0^{1} f( R_\gamma(s))\, {\mathrm d} s. \end{equation} (2.8)

    We also notice that (2.8) yields

    \begin{equation} \int_\gamma f = \int_X f\, {\mathrm d}\nu_\gamma\quad \text{where}\quad \nu_\gamma: = \ell(\gamma)( R_\gamma)_\sharp({\mathscr L}^{1} \llcorner {[0,1]}). \end{equation} (2.9)

    We will endow \mathrm{BVC}([0,1];X) with the compact-open topology \tau_ {\mathrm C} induced by \tau . By definition, a subbasis generating \tau_C is given by the collection of sets

    S(K,V): = \Big\{\gamma\in {\mathrm C}([0,1];X):\gamma(K)\subset V\Big\},\quad K\subset [0,1]\text{ compact,}\ V\text{ $\tau$-open in $X$.}

    By [19, §46, Thm. 46.8, 46.10] if the topology \tau is induced by a distance \delta , then the topology \tau_ {\mathrm C} is induced by the uniform distance \delta_ {\mathrm C}(\gamma,\gamma'): = \sup_{t\in [a,b]}\delta(\gamma(t),\gamma'(t)) and convergence w.r.t. the compact-open topology coincides with the uniform convergence w.r.t. \delta . If moreover \tau is separable then also \tau_ {\mathrm C} is separable [14, 4.2.18].

    We will denote by {\mathsf e}: \mathrm{BVC}([0,1];X)\times [0,1]\to X the evaluation map, which is defined by {\mathsf e}_t(\gamma) = {\mathsf e}(\gamma,t): = \gamma(t) for every t\in [0,1] ; {\mathsf e} is continuous. We will also adopt the notation {\mathsf e}[\gamma]: = {\mathsf e}(\{\gamma\}\times[0,1]) = \{\gamma(t):t\in [0,1]\} for the image of \gamma in X .

    The extended distance {\mathsf d}:X\times X\to[0,\infty] induces the extended distance {\mathsf d}_ {\mathrm C} in \mathrm{BVC}([0,1];X) by

    {\mathsf d}_ {\mathrm C}(\gamma_1,\gamma_2): = \sup\limits_{t\in [a,b]} {\mathsf d}(\gamma_1(t),\gamma_2(t))

    and ( {\mathrm C}([0,1];X),\tau_ {\mathrm C}, {\mathsf d}_ {\mathrm C}) is an extended metric-topological space [21, Prop. 3.2].

    Let us denote by \Sigma the set of continuous, nondecreasing and surjective maps \sigma:[0,1]\to[0,1] . On \mathrm{BVC}([0,1];X) we introduce the relation

    \begin{gathered} \gamma_1\sim \gamma_2\quad\text{if}\quad \exists\,\sigma_i\in \Sigma:\ {\gamma_1}\circ\sigma_1 = {\gamma_2}\circ\sigma_2, \end{gathered}

    and the function

    {\mathsf d}_ {\mathrm A}(\gamma_1,\gamma_2): = \inf\limits_{\sigma_i\in \Sigma} {\mathsf d}_ {\mathrm C}(\gamma_1\circ\sigma_1,\gamma_2\circ\sigma_2) \quad\text{ for every $ \gamma_i\in \mathrm{BVC}([0,1];X) $}.

    It is possible to prove that \sim is an equivalence relation [21, §3.2, Cor. 3.5] and {\mathsf d}_ {\mathrm A} satisfies

    \begin{align*} {\mathsf d}_ {\mathrm A}(\gamma_1,\gamma_2) = {\mathsf d}_ {\mathrm A}( R_{\gamma_1}, R_{\gamma_2}) & = \inf\limits_{\sigma\in \Sigma\atop \sigma\text{ injective}} {\mathsf d}_ {\mathrm C}({\gamma_1},{\gamma_2}\circ\sigma) = \inf\limits_{\gamma_i'\sim\gamma_i} {\mathsf d}_{ {\mathrm C}}(\gamma_1',\gamma_2'). \end{align*}

    In particular {\mathsf d}_ {\mathrm A} satisfies the triangle inequality, is invariant with respect to \sim and \gamma\sim\gamma' if and only if {\mathsf d}_ {\mathrm A}(\gamma,\gamma') = 0 . We collect a list of useful properties [21, §3.2]:

    Lemma 2.5. (a) The space \mathrm{RA}(X): = \mathrm{BVC}([0,1];X)/\sim endowed with the quotient topology \tau_ {\mathrm A} is an Hausdorff space. We will denote by \mathfrak{q}: \mathrm{BVC}([0,1];X)\to \mathrm{RA}(X) the quotient map.

    (b) If the topology \tau is induced by the distance \delta then the quotient topology \tau_ {\mathrm A} is induced by \delta_ {\mathrm A} (considered as a distance between equivalence classes of curves).

    (c) ( \mathrm{RA}(X),\tau_ {\mathrm A}, {\mathsf d}_ {\mathrm A}) is an extended metric-topological space.

    (d) For every \gamma,\gamma'\in \mathrm{BVC}([0,1];X) we have

    \gamma\sim\gamma'\quad\Leftrightarrow\quad R_{\gamma} = R_{\gamma'},

    and all the curves \gamma' equivalent to \gamma can be described as \gamma' = R_\gamma\circ\sigma for some \sigma\in \Sigma . Moreover, if \gamma'\sim\gamma then

    \ell(\gamma') = \ell(\gamma),\quad R_{\gamma'} = R_\gamma,\quad \int_\gamma f = \int_{\gamma'} f,

    so that the functions R , \ell , the evaluation maps {\mathsf e}_0, {\mathsf e}_1 , and the integral \int_\gamma f are invariant w.r.t. reparametrizations. We will still denote them by the same symbols.

    (e) \ell: \mathrm{RA}(X)\to [0,+\infty] is \tau_ {\mathrm A} -lower semicontinuous and {\mathsf e}_0, {\mathsf e}_1: \mathrm{RA}(X)\to X are continuous. If f:X\to[0,+\infty] is lower semicontinuous then the map \gamma\mapsto \int_\gamma f is lower semicontinuous w.r.t. \tau_ {\mathrm A} in \mathrm{RA}(X) .

    We conclude this section with a list of useful properties concerning the compactness in \mathrm{RA}(X) and the measurability of some importants maps, see [21, Thm. 3.13].

    Theorem 2.6. (a) If \gamma_i , i\in I , is a converging net in \mathrm{RA}(X) with \gamma = \lim_{i\in I}\gamma_i and \lim_{i\in I} \ell(\gamma_i) = \ell(\gamma) then

    \lim\limits_{i\in I} R_{\gamma_i} = R_\gamma\quad{w.r.t. }\;\tau_ {\mathrm C},

    and for every bounded and continuous function f\in {\mathrm C}_b(X,\tau) we have

    \lim\limits_{i\in I}\int_{\gamma_i}f = \int_\gamma f.

    In particular, we have

    \lim\limits_{i\in I}\nu_{\gamma_i} = \nu_\gamma\quad{weakly \;in }\; \mathcal M_+(X).

    (b) The map \gamma\mapsto \nu_\gamma from \mathrm{RA}(X) to \mathcal M_+(X) is universally Lusin measurable.

    (c) The map \gamma\mapsto R_\gamma is universally Lusin measurable from \mathrm{RA}(X) to \mathrm{BVC}_c([0,1];X) endowed with the topology \tau_ {\mathrm C} and it is also Borel if (X,\tau) is Souslin.

    (d) If f:X\to \mathbb{R} is a bounded Borel function (or f:X\to[0,+\infty] Borel) the map \gamma\mapsto \int_\gamma f is Borel. In particular the family of measures \{\nu_\gamma\}_{\gamma\in \mathrm{RA}(X)} is Borel.

    (e) If (X,\tau) is compact and \Gamma\subset \mathrm{RA}(X) satisfies \sup_{\gamma\in \Gamma} \ell(\gamma)<+\infty then \Gamma is relatively compact in \mathrm{RA}(X) w.r.t. the \tau_ {\mathrm A} topology.

    (f) If (X, {\mathsf d}) is complete and \Gamma\subset \mathrm{RA}(X) satisfies the following conditions:

    1) \sup_{\gamma\in \Gamma} \ell(\gamma)<+\infty ;

    2) there exists a \tau -compact set K\subset X such that {\mathsf e}[\gamma]\cap K\neq \emptyset for every \gamma\in \Gamma ;

    3) \{\nu_\gamma:\gamma\in \Gamma\} is equally tight, i.e. for every \varepsilon>0 there exists a \tau -compact set K_ \varepsilon\subset X such that \nu_\gamma(X\setminus K_ \varepsilon)\le \varepsilon for every \gamma\in \Gamma ,

    then \Gamma is relatively compact in \mathrm{RA}(X) w.r.t. the \tau_ {\mathrm A} topology.

    Notice that the third condition in the statement (f) of Theorem 2.6 implies the second one whenever \inf_{\gamma\in \Gamma}\ell(\gamma)>0 .

    In this section we will always assume that \mathbb{X} = (X,\tau, {\mathsf d}, \mathfrak m) is a complete e.m.t.m. space and \mathscr{A} is a compatible sub-algebra of \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) . We also fix a summability exponent p\in (1,\infty) with conjugate q = p/(p-1) .

    Let us first define the notion of Cheeger energy \mathsf{C\kern-1.5pt E}_{p} associated to \mathbb{X} , [3,5,6,10,21].

    Definition 3.1 (Cheeger energy). For every \kappa\ge0 and p\in (1,\infty) we define the "pre-Cheeger" energy functionals

    \mathsf{p\kern-1pt C\kern-1.5pt E}_{p}(f): = \int_X \big( \mathop{\rm{lip }}\nolimits f(x)\big)^p \, {\mathrm d} \mathfrak m, \qquad {for \;every } \; f\in \mathop{\rm{Lip }}\nolimits_b(X,\tau, {\mathsf d}),

    with \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f) = +\infty if f \in L^p(X)\setminus \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) . The L^p -lower semicontinuous envelope of \mathsf{p\kern-1pt C\kern-1.5pt E}_{p} is the "strong" Cheeger energy

    \mathsf{C\kern-1.5pt E}_{p}(f): = \inf \Big\{\liminf\limits_{n\to \infty}\int_X \big( \mathop{\rm{ lip }}\nolimits f_n\big)^p\, {\mathrm d} \mathfrak m: f_n\in \mathop{\rm{Lip }}\nolimits_b(X,\tau, {\mathsf d}),\ f_n\to f \;{ in }\;L^p(X, \mathfrak m)\Big\}.

    For every k\ge0 and f\in L^p(X, \mathfrak m) we also set

    \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}(f): = \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f)+ \kappa \|f\|_{L^p(X, \mathfrak m)}^p,\qquad \mathsf{C\kern-1.5pt E}_{p, \kappa}(f): = \mathsf{C\kern-1.5pt E}_p(f)+ \kappa \|f\|_{L^p(X, \mathfrak m)}^p.

    We denote by H^{1,p}( \mathbb{X}) the subset of L^p(X, \mathfrak m) whose elements f have finite Cheeger energy \mathsf{C\kern-1.5pt E}_{p}(f)<\infty : it is a Banach space with norm \|f\|_{ H^{1,p}( \mathbb{X})}: = \Big( \mathsf{C\kern-1.5pt E}_{p,1}(f)\Big)^{1/p}.

    Remark 3.2 (The notation \mathsf{C\kern-1.5pt E} and H^{1,p} ). We used the symbol \mathsf{C\kern-1.5pt E} instead of \mathsf{Ch} (introduced by [6]) in the previous definition to stress three differences:

    ● the dependence on the strongest \mathop{{\rm{lip}}}\nolimits f instead of | {\mathrm D} f| ,

    ● the factor 1 instead of 1/p in front of the energy integral.

    In this paper we will mainly adopt the "strong" approach to metric Sobolev spaces and we will use the notation H^{1,p}( \mathbb{X}) to stress this fact. We refer to [5,6] for the equivalent weak definition of W^{1,p}( \mathbb{X}) by test plan. In the final section 5 we will also use a few properties related to the intermediate (but still equivalent) Newtonian point of view, see [8,17].

    It is not difficult to check that \mathsf{C\kern-1.5pt E}_p:L^p(X, \mathfrak m)\to [0,+\infty] is a convex, lower semicontinuous and p -homogeneous functional; it is the greatest L^p -lower semicontinuous functional "dominated" by \mathsf{p\kern-1pt C\kern-1.5pt E}_{p} . Notice that when \mathfrak m has not full support, two different elements f_1,f_2\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) may give rise to the same equivalence class in L^p(X, \mathfrak m) . In this case, \mathsf{C\kern-1.5pt E}_p can be equivalently defined starting from the functional

    \widetilde{ \mathsf{p\kern-1pt C\kern-1.5pt E}}_{p}(f): = \inf\Big\{ \mathsf{p\kern-1pt C\kern-1.5pt E}_p(\tilde f):\tilde f\in \mathop{\rm{Lip }}\nolimits_b(X,\tau, {\mathsf d}),\ \tilde f = f \text{ $ \mathfrak m$-a.e.}\Big\},

    defined on the quotient space \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d})/\sim_{ \mathfrak m} .

    Whenever \mathsf{C\kern-1.5pt E}_p(f)<\infty one can show [5,6] that the closed convex set

    \begin{equation} S_p(f): = \Big\{G\in L^p(X, \mathfrak m): \exists f_n\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}):f_n\to f,\ \mathop{{\rm{lip}}}\nolimits f_n \rightharpoonup G\text{ in }L^p(X, \mathfrak m)\Big\} \end{equation} (3.1)

    admits a unique element of minimal norm, the minimal relaxed gradient denoted by | {\mathrm D} f|_\star . | {\mathrm D} f|_\star is also minimal in S_p(f) with respect to the natural order structure, i.e.,

    \begin{equation} | {\mathrm D} f|_\star\in S_p(f),\quad | {\mathrm D} f|_\star\le G\quad\text{for every }G\in S_p(f). \end{equation} (3.2)

    The Cheeger energy \mathsf{C\kern-1.5pt E}_{p} admits an integral representation in terms of the minimal relaxed gradient:

    \mathsf{C\kern-1.5pt E}_p(f) = \int_X | {\mathrm D} f|_\star^p(x)\, {\mathrm d} \mathfrak m(x)\quad \text{for every }f\in H^{1,p}( \mathbb{X}),

    and enjoys the following strong approximation result (see [5,6] in the case of bounded Lipschitz functions, [7] for the "metric" algebra generated by truncated distance functions and [21, Thm. 12.1] for the general case):

    Theorem 3.3 (Density in energy of compatible algebras). Let \mathscr{A} be a compatible sub-algebra of \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) and let I be a closed (possibly unbounded) interval of \mathbb{R} . For every f\in H^{1,p}( \mathbb{X}) taking values in I there exists a sequence (f_n)\subset \mathscr{A} with values in I such that

    f_n\to f,\quad \mathop{\rm{ lip }}\nolimits f_n\to | {\mathrm D} f|_\star\quad{strongly \;in }\;L^p(X, \mathfrak m).

    We collect a list of useful properties [6] of the minimal p -relaxed gradient.

    Theorem 3.4. For every f,g\in H^{1,p}( \mathbb{X}) we have

    (a) (Pointwise sublinearity) For | {\mathrm D} (\alpha f+\beta g)|_{\star}\le \alpha | {\mathrm D} f|_{\star}+\beta | {\mathrm D} g|_{\star} .

    (b) (Leibniz rule) For every f,g\in H^{1,p}( \mathbb{X})\cap L^\infty(X, \mathfrak m) we have fg\in H^{1,p}( \mathbb{X}) and

    \begin{equation} | {\mathrm D} {(fg)}|_\star\le |f|\,| {\mathrm D} g|_\star+|g|\,| {\mathrm D} f|_\star. \end{equation} (3.3)

    (c) (Locality) For any Borel set N\subset \mathbb{R} with {\mathscr L}^{1}(N) = 0 we have

    | {\mathrm D} f|_{\star} = 0\quad{ \mathfrak m-a.e. \;on }\;f^{-1}(N).

    In particular for every constant c\in \mathbb{R}

    \text{$ | {\mathrm D} f|_{\star} = | {\mathrm D} g|_{\star} \mathfrak m -a.e. \;on \; \{f-g = c\} $}.

    (d) (Chain rule) If \phi\in \mathop{{\rm{Lip}}}\nolimits( \mathbb{R}) then \phi\circ f\in H^{1,p}( \mathbb{X}) with

    \begin{equation} | {\mathrm D} (\phi\circ f)|_{\star}\le |\phi'(f)|\,| {\mathrm D} f|_{\star}. \end{equation} (3.4)

    Equality holds in (3.4) if \phi is monotone or {\mathrm C}^1 .

    Let us now study a few important properties of the Legendre transform of the p -Cheeger energy and its relation with the dual of the Sobolev space H^{1,p}( \mathbb{X}) when p\in (1,\infty) ; recall that we denote by q = p' = p/(p-1) the conjugate exponent of p . Let us first recall a simple property of p -homogeneous convex functionals (see e.g., [21, Lemma A.7].

    Lemma 3.5 (Dual of positively p -homogeneous functionals). Let C be a convex cone of some vector space V , p>1 , and \phi,\psi:C\to [0,\infty] with \psi = \phi^{1/p} , \phi = \psi^p . We have the following properties:

    (a) \phi is convex and positively p -homogeneous (i.e., \phi( \kappa v) = \kappa^p\phi(v) for every \kappa\ge 0 and v\in C ) in C if and only if \psi is convex and positively 1 -homogeneous on C (a seminorm, if C is a vector space and \psi is finite and even).

    (b) Under one of the above equivalent assumptions, setting for every linear functional z:V\to \mathbb{R}

    \begin{equation} \frac 1q\phi^*(z): = \sup\limits_{v\in C} {\langle} z,v {\rangle}-\frac 1p\phi(v),\quad \psi_*(z): = \sup\Big\{ {\langle} z,v {\rangle}:v\in C,\ \psi(v)\le 1\Big\}, \end{equation} (3.5)

    we have

    \psi_*(z) = \inf\Big\{c\ge 0: {\langle} z,v {\rangle}\le c\,\psi(v)\quad {for \;every }\;v\in C\Big\}, \quad \phi^*(z) = (\psi_*(z))^q,

    where in the first infimum we adopt the convention \inf A = +\infty if A is empty.

    (c) An element v\in C attains the first supremum in (3.5) if and only if

    \left\langle z,v \right\rangle = (\psi_*(z))^q = (\psi(v))^p.

    We want to study the dual functionals related to \mathsf{C\kern-1.5pt E}_{p, \kappa} and \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa} . The simplest situation is provided by L^p-L^q -duality:

    \begin{aligned} \frac1q \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(w): = &\sup\limits_{u\in L^p} \int_X wu\, {\mathrm d} \mathfrak m-\frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(u) \quad\text{for every }w\in L^q(X, \mathfrak m),\\ \frac1q \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*(w): = &\sup\limits_{u\in \mathop{\rm{Lip }}\nolimits_b(X,\tau, {\mathsf d})} \int_X wu\, {\mathrm d} \mathfrak m-\frac 1p \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}(u) \quad\text{for every }w\in L^q(X, \mathfrak m). \end{aligned}

    By Fenchel-Moreau duality Theorem (see e.g., [9, Theorem 1.11], [13, Chap. Ⅳ]) it is immediate to check that

    \begin{align} \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*(w)& = \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(w)\quad&\text{for every }w\in L^q(X, \mathfrak m),\\ \frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(u)& = \sup\limits_{w\in L^q(X, \mathfrak m)} \int_X uw\, {\mathrm d} \mathfrak m-\frac 1q \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^{*}(w) \quad&\text{for every }u\in L^p(X, \mathfrak m). \end{align} (3.6)

    The situation is more complicated if one wants to study the dual of \mathsf{C\kern-1.5pt E}_p with respect to the Sobolev duality. Just to clarify all the possibilities we consider three normed vector spaces:

    ● The separable and reflexive Banach space V: = L^p(X, \mathfrak m) ;

    ● The vector space \mathscr{A}_p associated to a compatible algebra \mathscr{A} endowed with the norm \mathsf{p\kern-1pt C\kern-1.5pt E}^{1/p}_{p,1} .

    ● The Banach space W = H^{1,p}( \mathbb{X}) with the norm \mathsf{C\kern-1.5pt E}^{1/p}_{p,1} .

    Notice that we do not know any information concerning the separability and the reflexivity of the Banach space H^{1,p}( \mathbb{X}) nor the (strong) density of \mathscr{A} in W . Since both \mathscr{A} and W = H^{1,p}( \mathbb{X}) are dense in V = L^p(X, \mathfrak m) , if we identify V' with L^q(X, \mathfrak m) we clearly have

    L^q(X, \mathfrak m) = V'\subset ( \mathscr{A}_p) ',\quad L^q(X, \mathfrak m) = V'\subset W'\quad \text{with continuous inclusions.}

    On the other hand, every element L\in W' can be considered as a bounded linear functional on \mathscr{A}_p and thus induces an element L_{\rm restr} of ( \mathscr{A}_p) ' just by restriction, but it may happen that this identification map is not injective. Finally, since \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1} may be strictly greater than \mathsf{C\kern-1.5pt E}_{p,1} on \mathscr{A}_p , in general not all the bounded linear functionals on \mathscr{A}_p may admit an extension to W .

    Taking all these facts into account, now we want to address the question of the unique extension of a given bounded linear functional L on \mathscr{A}_p to an element of the dual Sobolev space W' . We begin with a precise definition.

    Definition 3.6 (The spaces H^{-1,q}( \mathbb{X}) , H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) and \mathscr{A}_{q}' ). We define:

    H^{-1,q}( \mathbb{X}) as the dual W' of H^{1,p}( \mathbb{X}) ;

    H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) as the subset of H^{-1,q}( \mathbb{X}) whose elements L satisfy the following condition: for every choice of f,f_n\in H^{1,p}( \mathbb{X}) , n\in \mathbb{N} , and every constant C>0

    \begin{equation} \mathsf{C\kern-1.5pt E}_{p}(f_n)\le C,\quad \lim\limits_{n\to\infty}\|f_n-f\|_{L^p(X, \mathfrak m)} = 0\quad\Rightarrow\quad \lim\limits_{n\to\infty} {\langle} L,f_n {\rangle} = {\langle} L,f {\rangle} . \end{equation} (3.7)

    \mathscr{A}_{q}' as the set of linear functionals L on \mathscr{A} satisfying the following two conditions: there exists a constant D>0 such that

    \begin{equation} \big| {\langle} L,f {\rangle}\big|\le D\Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f)\Big)^{1/p}\quad{for \;every }\;f\in \mathscr{A}, \end{equation} (3.8a)

    and for every sequence f_n\in \mathscr{A} and every constant C>0

    \begin{equation} \mathsf{p\kern-1pt C\kern-1.5pt E}_{p}(f_n)\le C,\quad \lim\limits_{n\to\infty}\|f_n\|_{L^p(X, \mathfrak m)} = 0\quad\Rightarrow\quad \liminf\limits_{n\to\infty}\big| {\langle} L,f_n {\rangle}\big| = 0. \end{equation} (3.8b)

    When \mathscr{A} = \mathscr{A}( \mathbb{X}) = \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) we will write \mathscr{A}_{q}' = \mathscr{A}_q'( \mathbb{X}) .

    It would not be difficult to check that if H^{1,p}( \mathbb{X}) is reflexive then \mathscr{A} is strongly dense in H^{1,p}( \mathbb{X}) and H^{-1,q}( \mathbb{X}) = H^{-1,q}_{\mathrm{pd}}(\mathbb{X})\simeq \mathscr{A}_{q}' . In the general case, only a partial result holds and we will show that H^{1,p}( \mathbb{X}) can be identified with the dual of H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) , i.e., H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) is a predual of H^{1,p}( \mathbb{X}) (this property justifies the index _{\rm pd} in the notation). Let us start with a first identification:

    Proposition 3.7 ( \mathscr{A}_{q}'\simeq H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) ). The following properties hold:

    (a) \mathscr{A}_{q}' is a closed subspace of ( \mathscr{A}_p)' : in particular, it is a Banach space with the norm

    \| L\|_{ \mathscr{A}_{q}'}: = \Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*( L)\Big)^{1/q} = \sup\Big\{ {\langle} L,f {\rangle}:f\in \mathscr{A},\ \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f)\le 1\Big\}.

    (b) A linear functional L on \mathscr{A} belongs to \mathscr{A}_{q}' if and only if for every \varepsilon>0 there exists a constant \kappa>0 such that

    \begin{equation} \big| {\langle} L,f {\rangle}\big|^p\le \varepsilon\, \mathsf{p\kern-1pt C\kern-1.5pt E}_{p}(f)+\kappa \|f\|_{L^p}^p\quad\mathit{\text{for every}}\quad f\in \mathscr{A}. \end{equation} (3.9)

    In this case (3.8b) holds in the stronger form where \liminf is replaced by \limsup .

    (c) Every linear functional L\in \mathscr{A}_{q}' admits a unique extension \tilde L in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}). The map L\mapsto \tilde L is a surjective isometry between \mathscr{A}_{q}' and H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) , which is therefore a closed subspace of H^{-1,q}( \mathbb{X}) . In particular, if L, L'\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) coincide on \mathscr{A} then L = L' .

    Proof. (a) It is sufficient to prove that \mathscr{A}_{q}' is closed in the Banach space ( \mathscr{A}_p)' . Let L be an element of the closure and for every \varepsilon>0 choose elements L_ \varepsilon\in \mathscr{A}_{q}' such that \| L- L_ \varepsilon\|_{( \mathscr{A}_p)'}\le \varepsilon . For every sequence f_n\in \mathscr{A} as in (3.8b) we have

    \begin{align*} \liminf_{n\to\infty} \big| {\langle} L,f_n {\rangle}\big|\le \limsup\limits_{n\to\infty} \big| {\langle} L- L_ \varepsilon,f_n {\rangle}\big|+ \liminf_{n\to\infty}\big| {\langle} L_ \varepsilon,f_n {\rangle}\big|\le C \varepsilon. \end{align*}

    Since \varepsilon>0 is arbitrary we obtain that L\in \mathscr{A}_{q}' .

    (b) If L satisfies (3.9) and f_n\in \mathscr{A} is a sequence as in (3.8b) we have

    \begin{align*} \limsup\limits_{n\to\infty}\big| {\langle} L,f_n {\rangle}\big|^p \le \limsup\limits_{n\to\infty} \Big( \varepsilon\, \mathsf{p\kern-1pt C\kern-1.5pt E}_{p}(f_n)+ \kappa \|f_n\|_{L^p}^p\Big)\le \varepsilon\, C; \end{align*}

    since \varepsilon is arbitrary we deduce that \limsup\limits_{n\to\infty}\big| {\langle} L,f_n {\rangle}\big| = 0 , thus (3.8b) in the stronger form.

    In order to prove the converse implication, we argue by contradiction by assuming that there exists \varepsilon>0 and a sequence f_n\in \mathscr{A} such that

    \big| {\langle} L,f_n {\rangle}\big|^p\ge \varepsilon \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f_n)+n \|f_n\|_{L^p}^p \gt 0.

    By possibly replacing f_n with f_n\big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f_n)\big)^{-1/p} , it is not restrictive to assume that \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f_n) = 1 ; by (3.8a) we have for n> \varepsilon

    \varepsilon\le \varepsilon \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f_n)+n\|f_n\|_{L^p}^p\le \big| {\langle} L,f_n {\rangle}\big|^p\le D^p

    so that \lim_{n\to\infty}\|f_n\|_{L^p} = 0 but \liminf_{n\to\infty}\big| {\langle} L,f_n {\rangle}\big|\ge \varepsilon^{1/p}>0.

    (c) In order to define \tilde L we fix f\in H^{1,p}( \mathbb{X}) and any sequence f_n\in \mathscr{A} such that f_n\to f in L^p(X, \mathfrak m) with E^p: = \sup \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f_n)<\infty . By (3.9), for every \varepsilon>0 there exists \kappa>0 such that

    | {\langle} L,f_n-f_m {\rangle}|\le 2 \varepsilon^{1/p} E+ \kappa^{1/p} \|f_n-f_m\|_{L^p}

    which shows that the sequence n\mapsto {\langle} L,f_n {\rangle} satisfies the Cauchy condition and thus admits a limit which we denote by {\langle} \tilde L,f {\rangle} . This notation is justified by the fact that the limit does not depend on the sequence f_n : in fact, if f_n' is another sequence converging to f in L^p(X, \mathfrak m) with equibounded energy, (3.7) shows that \lim\limits_{n\to\infty} {\langle} L,f_n-f_n' {\rangle} = 0 . It is also easy to check that the map H^{1,p}( \mathbb{X})\ni f\mapsto {\langle} \tilde L,f {\rangle} is a linear functional.

    In order to show that \tilde L is bounded, for every f\in H^{1,p}( \mathbb{X}) we select an optimal sequence f_n such that \mathsf{C\kern-1.5pt E}_p(f) = \lim_{n\to\infty} \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f_n) : by construction

    \big| {\langle} \tilde L,f {\rangle}\big| = \lim\limits_{n\to\infty} \big| {\langle} L,f_n {\rangle}\big|\le \limsup\limits_{n\to\infty}\| L\|_{ \mathscr{A}_{q}'} \Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f_n)\Big)^{1/p} = \| L\|_{ \mathscr{A}_{q}'}\Big( \mathsf{C\kern-1.5pt E}_{p,1}(f)\Big)^{1/p}

    so that \|\tilde L\|_{ H^{-1,q}( \mathbb{X})}\le \| L\|_{ \mathscr{A}_{q}'} . On the other hand for every f\in \mathscr{A} with \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f)\le 1 by choosing the constant sequence f_n\equiv f we get

    {\langle} L,f {\rangle} = {\langle} \tilde L,f {\rangle} = \|\tilde L\|_{ H^{-1,q}( \mathbb{X})}\Big( \mathsf{C\kern-1.5pt E}_{p,1}(f)\Big)^{1/p} \le \|\tilde L\|_{ H^{-1,q}( \mathbb{X})}

    since \mathsf{C\kern-1.5pt E}_p(f)\le \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f)\le 1 . It follows that \| L\|_{ \mathscr{A}_{q}'}\le \|\tilde L\|_{ H^{-1,q}( \mathbb{X})} so that the extension map \iota: L\mapsto \tilde L is an isometry.

    It remains to prove that the image of \iota coincides with H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) . Since it is clear that H^{-1,q}_{\mathrm{pd}}(\mathbb{X})\subset \iota( \mathscr{A}_{q}') , it is sufficient to show the converse inclusion, i.e., that every element \tilde L = \iota( L) satisfies (3.7). By linearity, it is not restrictive to check (3.7) for f = 0 . If f_n\in H^{1,p}( \mathbb{X}) has equibounded Cheeger energy and \lim_{n\to\infty}\|f_n\|_{L^p} = 0 , by the very definition of the Cheeger energy and the definition of \tilde L we can find another sequence g_n\in \mathscr{A} such that

    \mathsf{p\kern-1pt C\kern-1.5pt E}_p(g_n)\le \mathsf{C\kern-1.5pt E}_p(f_n)+\frac 1n, \quad \|g_n-f_n\|_{L^p}\le \frac 1n,\quad \big| {\langle} L,g_n {\rangle}- {\langle} \tilde L,f_n {\rangle}\big|\le \frac 1n.

    Since L\in \mathscr{A}_{q}' and \lim_{n\to\infty}\|g_n\|_{L^p} = 0 we have \lim_{n\to\infty} {\langle} L,g_n {\rangle} = 0 so that \lim_{n\to\infty} {\langle}\tilde L,f_n {\rangle} = 0 .

    Let us now express the dual functionals by a infimal convolution.

    Lemma 3.8. For every L\in H^{-1,q}( \mathbb{X}) and every \alpha \ge0 , \beta>0 we have

    \begin{equation} \begin{aligned} \frac1q \mathsf{C\kern-1.5pt E}_{p,\alpha+\beta}^*( L) & = \sup\limits_{g\in H^{1,p}( \mathbb{X})} {\langle} L,g {\rangle}-\frac 1p \mathsf{C\kern-1.5pt E}_{p,\alpha}(g)-\frac {\beta} p\|g\|_{L^p}^p \\& = \min_{f\in L^q(X, \mathfrak m)} \frac 1q \mathsf{C\kern-1.5pt E}_{p,\alpha}^*( L-f)+ \frac 1{q\beta^{q/p} }\|f\|_{L^q}^q. \end{aligned} \end{equation} (3.10)

    Proof. (3.10) is a particular case of the duality formula for the sum of two convex functions \varphi,\psi:W\to (-\infty,+\infty]

    (\varphi+\psi)^*( L) = \min_{f\in W}\varphi^*( L-f)+\psi^*(f)\quad \text{for every } L\in W'

    which holds in every Banach space W whenever there exists a point w_0\in W such that \phi(w_0)<\infty and \psi is finite and continuous at w_0 by Fenchel-Rockafellar Theorem ([20], see also [9, Theorem 1.12]). Here W = H^{1,p}( \mathbb{X}) , \phi(g): = \frac 1p \mathsf{C\kern-1.5pt E}_{p,\alpha}(g) , \psi(g): = \frac \beta p\|g\|_{L^p}^p .

    We collect in the next proposition a further list of useful properties. We will denote by {\mathrm J}_p:L^p(X, \mathfrak m)\to L^q(X, \mathfrak m) the duality map

    ( {\mathrm J}_p u)(x): = |u(x)|^{p-1}u(x),\quad \int_X u\, {\mathrm J}_p u\, {\mathrm d} \mathfrak m = \|u\|_{L^p}^p = \| {\mathrm J}_pu\|_{L^q}^q,

    and by {\mathrm A}_p: H^{1,p}( \mathbb{X})\to \mathfrak P( H^{-1,q}( \mathbb{X})) the subdifferential of the Cheeger energy with respect to the Sobolev duality

    \begin{equation} L\in {\mathrm A}_p u\quad\Leftrightarrow\quad u\in H^{1,p}( \mathbb{X}),\quad {\langle} L,v-u {\rangle}\le \frac 1p \mathsf{C\kern-1.5pt E}_p(v)-\frac 1p \mathsf{C\kern-1.5pt E}_p(u)\quad \text{for every }v\in H^{1,p}( \mathbb{X}). \end{equation} (3.11)

    Since \mathsf{C\kern-1.5pt E}_{p} is continuous in H^{1,p}( \mathbb{X}) , {\mathrm A}_pu\neq\emptyset for every u\in H^{1,p}( \mathbb{X}) [13, Chap. 1, Prop. 5.3] (notice that {\mathrm A}_p is different from the subdifferential of \mathsf{C\kern-1.5pt E}_{p} w.r.t. the L^p - L^q duality pair). The sum

    {\mathrm Q}_{p, \kappa}: = {\mathrm A}_p+ \kappa {\mathrm J}_p\quad\text{is the subdifferential in $ H^{1,p}( \mathbb{X}) $ of $ \frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa} $.}

    Proposition 3.9. We have the following properties

    (a) For every L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) and every \kappa\ge0 we have

    \begin{align} \frac1q \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L) = &\sup\limits_{f\in H^{1,p}( \mathbb{X})} {\langle} L,f {\rangle}-\frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(f) \\ = & \sup\limits_{\phantom{f\ \,\,}f\in \mathscr{A}\phantom{( \mathbb{X})}} {\langle} L,f {\rangle}-\frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(f) \end{align} (3.12)
    \begin{align} = & \sup\limits_{\phantom{f\ \,\,}f\in \mathscr{A}\phantom{( \mathbb{X})}} {\langle} L,f {\rangle}-\frac 1p \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}(f) = \frac 1q \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*( L). \end{align} (3.13)

    (b) H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) coincides with the (strong) closure of V' = L^q(X, \mathfrak m) in W' = H^{-1,q}( \mathbb{X}) .

    (c) For every L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) and \kappa>0 there exists a unique solution u_ \kappa = {\mathrm Q}_{p, \kappa}^{-1}( L)\in H^{1,p}( \mathbb{X}) of the problem

    \begin{equation} \min\limits_{u\in H^{1,p}( \mathbb{X})} \frac 1p \mathsf{C\kern-1.5pt E}_{p,\kappa}(u) - {\langle} L,u {\rangle} \end{equation} (3.14)

    which satisfies

    \begin{equation} {\mathrm Q}_{p, \kappa}u_ \kappa = {\mathrm A}_p u_ \kappa+ \kappa {\mathrm J}_p u_ \kappa\ni L,\quad \mathsf{C\kern-1.5pt E}^*_{p, \kappa}( L) = \mathsf{C\kern-1.5pt E}_{p, \kappa}(u_ \kappa) = {\langle} L,u_ \kappa {\rangle}. \end{equation} (3.15)

    (d) For every L\in H^{-1,q}( \mathbb{X}) and \kappa>0 there exists a unique function f_ \kappa: = {\mathrm R}_ \kappa(L) solving the minimum problem

    \begin{align} \frac 1q \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L) & = \min_{f\in L^q(X, \mathfrak m)} \frac 1q \mathsf{C\kern-1.5pt E}_p^*( L-f)+\frac 1{q \kappa^{q/p}}\|f\|_{L^q}^q \end{align} (3.16)

    The map {\mathrm R}_ \kappa: H^{-1,q}( \mathbb{X})\to L^q(Z, \mathfrak m) is strongly continuous. Moreover, if L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) then f_ \kappa = {\mathrm R}_\kappa(L) = \kappa {\mathrm J}_p u_ \kappa = \kappa {\mathrm J}_p\circ {\mathrm Q}_{p, \kappa}^{-1}( L).

    (e) For every L\in H^{-1,q}( \mathbb{X}) we have

    \mathsf{C\kern-1.5pt E}_p^*( L) = \lim\limits_{ \kappa \downarrow0} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L) = \sup\limits_{ \kappa \gt 0} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L).

    Proof. (a) (3.13) (which implies (3.12)) follows by an easy approximation argument combining the definition of \mathsf{C\kern-1.5pt E}_p and the continuity property (3.7) and it follows by the same argument at the end of the proof of claim (c) of Proposition 3.7.

    (b) Since H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) is a closed subspace of H^{-1,q}( \mathbb{X}) and clearly contains L^q(X, \mathfrak m) , it is sufficient to prove that L^q(X, \mathfrak m) is dense in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) . For every n\in \mathbb{N} we consider the functional {\mathsf G}_n: = \mathsf{C\kern-1.5pt E}_{p,1+n^p}^* and we want to show that

    \begin{equation} \limsup\limits_{n \uparrow\infty} {\mathsf G}_n( L) = 0; \end{equation} (3.17)

    by using (3.10) (with \alpha: = 1,\beta: = n^p ), (3.17) is in fact equivalent to the density of L^q(X, \mathfrak m) in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) . By the first formula of (3.10), for every \varepsilon>0 we can find g_n\in H^{1,p}( \mathbb{X}) such that

    \begin{equation} \frac 1q {\mathsf G}_n( L)\le {\langle} L,g_n {\rangle}- \frac1p \mathsf{C\kern-1.5pt E}_{p,1}(g_n)-\frac {n^p}{p} \|g_n\|_{L^p}^p+ \varepsilon. \end{equation} (3.18)

    Since

    {\langle} L,g_n {\rangle}\le \frac{2^{q/p}}{q} \mathsf{C\kern-1.5pt E}_{p,1}^*( L)+\frac1{2p} \mathsf{C\kern-1.5pt E}_{p,1}(g_n)

    and {\mathsf G}_n( L)\ge0 , we obtain

    \frac1{2p} \mathsf{C\kern-1.5pt E}_{p,1}(g_n) +\frac {n^p}{p} \|g_n\|_{L^p}^p\le \varepsilon+ \frac{2^{q/p}}{q} \mathsf{C\kern-1.5pt E}_{p,1}^*( L)

    so that \mathsf{C\kern-1.5pt E}_{p,1}(g_n) is uniformly bounded and \|g_n\|_{L^p}\to0 as n\to\infty . By (3.7) we conclude that \lim_{n\to\infty} {\langle} L,g_n {\rangle} = 0 and therefore (3.18) yields \limsup_{n\to\infty} {\mathsf G}_n( L)\le \varepsilon . Since \varepsilon>0 is arbitrary, we obtain (3.17).

    (c) The existence of a solution u_ \kappa\in H^{1,p}( \mathbb{X}) to (3.14) follows by (3.7) and the Direct method of the Calculus of Variations. Let us take a minimizing sequence f_n\in H^{1,p}( \mathbb{X}) such that

    \begin{equation} \lim\limits_{n\to\infty} \frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(f_n)- {\langle} L,f_n {\rangle} = M: = \inf\limits_{f\in H^{1,p}( \mathbb{X})}\frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(f_n)- {\langle} L,f_n {\rangle}. \end{equation} (3.19)

    Since f_n is uniformly bounded in H^{1,p}( \mathbb{X}) , up to extracting a suitable subsequence (still denoted by f_n ), it is not restrictive to assume that f_n is converging to a function f\in H^{1,p}( \mathbb{X}) weakly in L^p(X, \mathfrak m) and

    \begin{equation} S = \lim\limits_{n\to\infty}\|f_n\|_{L^p} = \lim\limits_{n\to\infty}\Big[\frac p \kappa \Big(M + {\langle} L,f_n {\rangle}- \frac 1p \mathsf{C\kern-1.5pt E}_{p}(f_n)\Big)\Big]^{1/p}. \end{equation} (3.20)

    We prove that f_n is a Cauchy sequence: by the uniform convexity of the L^p(X, \mathfrak m) -norm, for every \varepsilon>0 there exist S'<S<S'' such that for every h_1,h_2\in L^p(X, \mathfrak m)

    \begin{equation} \|h_1\|_{L^p}\le S'',\ \|h_2\|_{L^p}\le S'',\quad \left\|\frac{h_1+h_2}2\right\|_{L^p}\ge S'\quad\Rightarrow \quad \|h_2-h_1\|\le \varepsilon. \end{equation} (3.21)

    By (3.20) we can find \bar n\in \mathbb{N} such that for every n\ge \bar n and

    \|f_n\|_{L^p}\le S'',\qquad M-\frac 1p \mathsf{C\kern-1.5pt E}_p(f_n)+ {\langle} L,f_n {\rangle}\ge \frac \kappa p (S')^p.

    For every m,n\ge \bar n we thus get

    \begin{align*} M&\le \frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}\Big(\frac 12(f_n+f_m)\Big)-\frac12 {\langle} L,f_n+f_m {\rangle} \\&\le \frac 12\Big(\frac1p \mathsf{C\kern-1.5pt E}_p(f_n)- {\langle} L,f_n {\rangle}+ \frac1p \mathsf{C\kern-1.5pt E}_p(f_m)- {\langle} L,f_m {\rangle}\Big) +\frac\kappa p\Big\|\frac{f_n+f_m}2\Big\|_{L^p}^p \\&\le M-\frac \kappa p (S')^p+\frac\kappa p\Big\|\frac{f_n+f_m}2\Big\|_{L^p}^p, \end{align*}

    and therefore

    \Big\|\frac{f_n+f_m}2\Big\|_{L^p}\ge S'

    so that (3.21) yields \|f_n-f_m\|_{L^p}\le \varepsilon for every n,m\ge \bar n . We deduce that \lim_{n\to\infty} \|f_n-f\|_{L^p} = 0; since f_n is uniformly bounded in H^{1,p}( \mathbb{X}) , (3.7) yields \lim_{n\to\infty } {\langle} L,f_n {\rangle} = {\langle} L,f {\rangle} and the lower semicontinuity of the Cheeger energy yields \mathsf{C\kern-1.5pt E}_p(f)\le \liminf_{n\to\infty} \mathsf{C\kern-1.5pt E}_p(f_n) . By (3.19) we conclude that \frac1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(f)- {\langle} L,f {\rangle} = M so that f is the unique minimizer of (3.14).

    (d) (3.16) is an immediate consequence of (3.10) with \alpha = 0 and \beta = \kappa .

    In order to prove the continuity of {\mathrm R}_ \kappa , let L_n\in H^{-1,q}( \mathbb{X}) be a sequence strongly converging to L and let f_n = {\mathrm R}_ \kappa(L_n)\in L^q(X, \mathfrak m) . Since \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_n) is uniformly bounded, we obviously get a uniform bound for \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_n-f_n) and \|f_n\|_{L^q} . Let f\in L^q(X, \mathfrak m) be any weak L^q limit point of f_n , e.g., attained along a subsequence f_{n(j)} . Since \lim_{n\to\infty} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_n) = \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L) and

    \liminf\limits_{j\to\infty} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_{n(j)}-f_{n(j)})\ge \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L-f),\qquad \liminf\limits_{j\to\infty}\|f_{n(j)}\|_{L^q}^q\ge \|f\|_{L^q}^q

    we deduce that

    \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L)\ge \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L-f)+\frac1{q \kappa^{q/p}}\|f\|_{L^q}^q

    so that f = {\mathrm R}_ \kappa(L) . Since {\mathrm R}_ \kappa(L) is the unique weak limit point of the sequence f_n in L^q , we conclude that f_n \rightharpoonup {\mathrm R}_ \kappa(L) in L^q(X, \mathfrak m) . The same variational argument also shows that \limsup_{n\to\infty}\|f_n\|_{L^q}\le \|f\|_{L^q} so that the convergence is strong.

    Finally, if L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) , f_ \kappa is the (unique) minimizer of (3.10) and u_ \kappa is the (unique) minimizer of (3.14), we get

    \begin{align*} \frac1q \mathsf{C\kern-1.5pt E}_{p}( L-f_ \kappa)+\frac 1p \mathsf{C\kern-1.5pt E}_p(u_ \kappa) - {\langle} L-f_ \kappa,u_ \kappa {\rangle}+ \frac1{q \kappa^{q/p}}\|f_ \kappa\|_{L^q}^q+ \frac \kappa p\|u_ \kappa\|_{L^p}^p- {\langle} f_ \kappa,u_ \kappa {\rangle} = 0 \end{align*}

    which yields

    {\mathrm A}_p u_ \kappa = L-f_ \kappa,\quad f_ \kappa = \kappa {\mathrm J}_pu_ \kappa.

    (e) Since the map \kappa \mapsto \mathsf{C\kern-1.5pt E}^*_{p, \kappa}( L) is nonincreasing, we have \lim\limits_{ \kappa \downarrow0} \mathsf{C\kern-1.5pt E}^*_{p, \kappa}( L) = \sup\limits_{ \kappa>0} \mathsf{C\kern-1.5pt E}^*_{p, \kappa}( L)\le \mathsf{C\kern-1.5pt E}_p^*( L) . On the other hand, for every f\in H^{1,p}( \mathbb{X}) and \varepsilon>0 , choosing \kappa>0 sufficiently small so that \frac \kappa p\|f\|^p_{L^p}\le \varepsilon we get

    \begin{align*} {\langle} L, f {\rangle}-\frac 1p \mathsf{C\kern-1.5pt E}_p(f)- \varepsilon&\le {\langle} L, f {\rangle}-\frac 1p \mathsf{C\kern-1.5pt E}_p(f)-\frac \kappa p\|f\|^p_{L^p} = {\langle} L, f {\rangle}-\frac 1p \mathsf{C\kern-1.5pt E}_{p, \kappa}(f) \\& \le \frac 1q \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L) \le \frac 1q \sup\limits_{ \kappa \gt 0} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L) \end{align*}

    Since the inequality holds for every \varepsilon>0 and every f\in H^{1,p}( \mathbb{X}) , we obtain the converse inequality \mathsf{C\kern-1.5pt E}_p^*( L)\le \sup_{ \kappa>0} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*( L) .

    Proposition 3.9 yields the following interesting duality result, which is also related to the theory of derivations discussed in [12].

    Corollary 3.10 ( H^{1,p}( \mathbb{X}) is the dual of a Banach space). H^{1,p}( \mathbb{X}) can be isometrically identified with the dual of H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) . In particular, if L^q(X, \mathfrak m) is a separable space, H^{1,p}( \mathbb{X}) is the dual of a separable Banach space.

    Proof. Let z be a bounded linear functional on H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) . Since L^q(X, \mathfrak m) is continuously and densely imbedded in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) , for every f\in L^q(X, \mathfrak m) {\langle} z,f {\rangle}\le \|z\|_{ H^{-1,q}_{\mathrm{pd}}(\mathbb{X})}\|f\|_{L^q} , so that there exists a unique u = \iota(\zeta)\in L^p(X, \mathfrak m) such that

    {\langle} z,f {\rangle} = \int_X u f\, {\mathrm d} \mathfrak m\quad\text{for every }f\in L^q(X, \mathfrak m).

    By (3.6) and the strong density of L^q(X, \mathfrak m) in H^{-1,q}_{\mathrm{pd}}(\mathbb{X})

    \begin{align*} \frac1p \mathsf{C\kern-1.5pt E}_{p,1}(u) & = \sup\limits_{f\in L^q(X, \mathfrak m)}\int_X uf\, {\mathrm d} \mathfrak m- \frac 1q \mathsf{C\kern-1.5pt E}_{p,1}^*(f) = \sup\limits_{f\in L^q(X, \mathfrak m)} {\langle} z,f {\rangle}- \frac 1q\|f\|_{H^{-1,q}( \mathbb{X})}^q \\& = \sup\limits_{f\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X})} {\langle} z,f {\rangle}- \frac 1q\|f\|_{ H^{-1,q}_{\mathrm{pd}}(\mathbb{X})}^q = \frac 1p\|z\|_{( H^{-1,q}_{\mathrm{pd}}(\mathbb{X}))'}^p. \end{align*}

    It follows that \iota is an isometry from the dual of H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) and H^{1,p}( \mathbb{X}) . Since \iota is clearly surjective, we conclude.

    Remark 3.11 ( H^{1,p}( \mathbb{X}) as Gagliardo completion [16]). Recall that if (A,\|\cdot\|_A) is a normed vector space continuously imbedded in a Banach space (V,\|\cdot\|_V) , the Gagliardo completion A^{V,c} is the Banach space defined by

    W: = \Big\{v\in V:\exists (a_n)_n\subset A, \ \lim\limits_{n\to\infty}\|a_n-v\|_V = 0,\ \sup\limits_n \|a_n\|_A \lt \infty\Big\}

    with norm

    \|v\|_{W}: = \inf\Big\{\liminf\limits_{n\to\infty} \|a_n\|_A:a_n\in A,\ \lim\limits_{n\to\infty}\|a_n-v\|_V = 0\Big\}.

    When \mathop{{\rm{supp}}}\nolimits( \mathfrak m) = X , we can identify \mathscr{A} with a vector space A with the norm induced by \mathsf{p\kern-1pt C\kern-1.5pt E}_{p} imbedded in V: = L^p(X, \mathfrak m) ; it is immediate to check that H^{1,p}( \mathbb{X}) coincides with the Gagliardo completion of A in V . When A (and therefore W ) is strongly dense in V , we can identify the dual V' of V as a subset of the dual W' of W and we can define the set W_{\kern-2pt\mathrm{pd}}' as the closure of V' in W' . If V is uniformly convex, the same statements and characterizations given in Propositions 3.7 and 3.9 hold in this more abstract setting. In particular, W can be isometrically identified with the dual of W_{\kern-2pt\mathrm{pd}}' .

    The following result provides a useful criterion to check if a linear functional on \mathscr{A} belongs to \mathscr{A}_{q}' . Let us first recall that a subset F\subset L^1(X, \mathfrak m) is weakly relatively compact in L^1(X, \mathfrak m) if and only if it satisfies one of the following equivalent properties [13, Chap. Ⅷ, Theorem 1.3]:

    a) for all \varepsilon>0 there exists m\ge0 such that

    \int_{|f(x)|\ge m}|f(x)|\, {\mathrm d} \mathfrak m(x)\le \varepsilon\quad\text{for every }f\in F;

    b) (Equiintegrability) For all \varepsilon>0 there exists \delta>0 such that for every Borel set B\subset X

    \int_{B}|f(x)|\, {\mathrm d} \mathfrak m(x)\le \varepsilon\quad\text{whenever $f\in F$ and $ \mathfrak m(B)\le \delta$}.

    c) (Uniform superlinear estimate) There exists a positive, increasing, l.s.c. and convex function \Phi:[0,\infty)\to [0,\infty) such that \lim_{r\to\infty}\Phi(r)/r = +\infty and

    \sup\limits_{f\in F} \int_X \Phi(|f(x)|)\, {\mathrm d} \mathfrak m(x) \lt \infty.

    Proposition 3.12. Let L be a linear functional on \mathscr{A} satisfying (3.8a). If for every sequence f_n\in \mathscr{A} satisfying

    \begin{equation} -1\le f_n\le 1,\quad \lim\limits_{n\to\infty}\|f_n\|_{L^p(X, \mathfrak m)} = 0,\quad \{( \mathop{{\rm{lip}}}\nolimits f_n)^p:n\in \mathbb{N}\}\quad\mathit{\text{is equiintegrable}} \end{equation} (3.22)

    one has \liminf_{n\to\infty}| {\langle} L,f_n {\rangle}| = 0 , then L\in \mathscr{A}_{q}' .

    Proof. We split the proof in two steps.

    Claim 1: if L is a linear functional on \mathscr{A} satisfying (3.8a) and for every sequence ( f_n)_{n\in \mathbb{N}}\subset \mathscr{A}

    \begin{equation} \lim\limits_{n\to\infty}\|f_n\|_{L^p} = 0\ {and}\ \{( \mathop{{\rm{lip}}}\nolimits f_n)^p:n\in \mathbb{N}\}\;{ equiintegrable} \quad\Rightarrow\quad \liminf\limits_{n\to\infty}| {\langle} L,f_n {\rangle}| = 0 \end{equation} (3.23)

    then L\in \mathscr{A}_{q}' .

    We argue by contradiction and we assume that there exists a sequence f_n\in \mathscr{A} such that

    \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f_n)\le C,\quad \lim\limits_{n\to\infty}\|f_n\|_{L^p} = 0\quad\text{and}\quad \liminf\limits_{n\to\infty}| {\langle} L,f_n {\rangle}| \gt 0.

    By possibly changing the sign of f_n it is not restrictive to assume that {\langle} L,f_n {\rangle}\ge c>0 for every n\in \mathbb{N} . Applying Mazur Lemma we find coefficients \alpha_{n,m}\ge0 , n\in \mathbb{N} , 0\le m\le M(n) such that g_n: = \sum_{m = 0}^{M(n)} \alpha_{n,m} \mathop{{\rm{lip}}}\nolimits f_{n+m} is strongly converging in L^p(X, \mathfrak m) . Thus n\mapsto g_n^p is strongly converging in L^1(X, \mathfrak m) and it is therefore equi-integrable.

    We now consider \tilde f_n: = \sum_{m = 0}^{M(n)} \alpha_{n,m} f_{n+m} . By construction

    \begin{equation} {\langle} L,\tilde f_n {\rangle} = \sum\limits_{m = 0}^{M(n)} \alpha_{n,m} {\langle} L,f_{n+m} {\rangle}\ge c \gt 0,\quad \lim\limits_{n\to\infty}\|\tilde f_n\|_{L^p} = 0 \end{equation} (3.24)

    and

    \mathop{\rm{ lip }}\nolimits\tilde f_n\le \sum\limits_{m = 0}^{M(n)} \alpha_{n,m} \mathop{\rm{ lip }}\nolimits f_{n+m} = g_n

    so that (3.23) yields \liminf_{n\to\infty} {\langle} L,\tilde f_n {\rangle} = 0 , which contradicts the first inequality of (3.24).

    Claim 2: it is sufficient to prove the implication (3.23) for sequences taking values in [-1,1] . Let f_n\in \mathscr{A} as in (3.23), m_n: = \sup |f_n| , E^p: = \sup\limits_n \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f_n)<\infty , and let \phi\in {\mathrm C}^1( \mathbb{R}) be an odd function such that

    \begin{equation} \phi(r) = r\quad \text{if }|r|\le 1/2,\quad -1\le \phi(r)\le 1,\quad 0\le \phi'(r)\le 1\quad\text{for every }r\in \mathbb{R}. \end{equation} (3.25)

    Let us fix \varepsilon>0 and \delta: = \varepsilon/3E so that E^p\delta^p\le \varepsilon^p/3 . For every choice of n\in \mathbb{N} we can find an odd polynomial P_n such that (see e.g., [21, Lemma 2.23])

    -1\le P_n(r)\le 1,\quad 0\le P_n'(r)\le 1,\quad |P_n'(r)-\phi'(r)|\le \delta\quad \text{for every }r\in [-m_n,m_n],

    We set

    g_n: = P_n\circ f_n,\quad h_n: = f_n-g_n = Q_n(f_n)\quad\text{where}\quad Q_n(r) = r-P_n(r);

    notice that g_n and h_n belong to \mathscr{A} as well. Since \mathop{{\rm{lip}}}\nolimits g_n\le \mathop{{\rm{lip}}}\nolimits f_n and g_n takes values in [-1,1] , by assumption we have \liminf_{n\to\infty}| {\langle} L,g_{n} {\rangle}| = 0 . On the other hand, \|h_{n}\|_{L^p}\le \|f_n\|_{L^p} , \mathop{{\rm{lip}}}\nolimits h_{n}\le \mathop{{\rm{lip}}}\nolimits f_n , and \mathop{{\rm{lip}}}\nolimits h_{n}(x)\le Q_n'(f_n(x)) \mathop{{\rm{lip}}}\nolimits f_n(x)\le \delta \mathop{{\rm{lip}}}\nolimits f_n(x) whenever |f_n(x)|<1/2 . Since Chebichev inequality yields

    \mathfrak m\big\{|f_n|\ge 1/2\big\}\le 2^p\|f_n\|^p_{L^p(X, \mathfrak m)},\quad \lim\limits_{n\to\infty} \mathfrak m\big\{f_n\ge 1/2\big\} = 0,

    we can choose n_0 sufficiently big so that for every n\ge n_0

    \int_{\{|f_n|\ge 1/2\}} ( \mathop{\rm{ lip }}\nolimits f_n)^p\, {\mathrm d} \mathfrak m\le \varepsilon^p/3,\quad \int_X |h_n|^p\, {\mathrm d} \mathfrak m\le \varepsilon^p/3,

    and

    \begin{align*} \int_X ( \mathop{{\rm{lip}}}\nolimits h_n)^p\, {\mathrm d} \mathfrak m &\le \delta^p\int_{\{|f_n| \lt 1/2\}} ( \mathop{{\rm{lip}}}\nolimits f_n)^p\, {\mathrm d} \mathfrak m +\int_{\{|f_n|\ge 1/2\}} ( \mathop{{\rm{lip}}}\nolimits f_n)^p\, {\mathrm d} \mathfrak m \le \delta^p E^p+\frac 13 \varepsilon^p\le \frac 23 \varepsilon^p. \end{align*}

    (3.8a) then yields | {\langle} L,h_n {\rangle}|\le D \varepsilon for n\ge n_0 and therefore \liminf_{n\to\infty}| {\langle} L,f_n {\rangle}|\le D \varepsilon . Since \varepsilon>0 is arbitrary, we conclude.

    Our last criterium concerns positive functionals, i.e., satisfying

    \begin{equation} {\langle} L,f {\rangle}\ge0\quad\text{whenever }f\in \mathscr{A},\ f\ge0. \end{equation} (3.26)

    We will see in Theorem 4.7 that they are always induced by a Radon measure.

    Theorem 3.13. If L is a linear functional on \mathscr{A} satisfying (3.8a) and (3.26), then L\in \mathscr{A}_{q}' .

    Proof. We apply Proposition 3.12 and refine the last argument of its proof. Let f_n\in \mathscr{A} as in (3.22) with E^p: = \sup\limits_n \mathsf{C\kern-1.5pt E}_p(f_n) We select strictly positive parameters \varepsilon,\kappa>0 , \delta: = \varepsilon/3E , the odd function \phi as in (3.25) with \phi_\kappa(r): = \kappa \phi(r/\kappa) , and odd polynomials \tilde P_{\kappa, \varepsilon} satisfying

    -1\le \tilde P_{\kappa, \varepsilon}(r)\le 1,\quad 0\le \tilde P_{\kappa, \varepsilon}'(r)\le 1,\quad |\tilde P_{\kappa, \varepsilon}'(r)-\phi'(r)|\le \delta\quad \text{if }|r|\le \frac 1\kappa.

    We also set P_{\kappa, \varepsilon}(r): = \kappa \tilde P_{ \kappa, \varepsilon}(r/\kappa) , Q_{\kappa, \varepsilon}(r) = r-P_{\kappa, \varepsilon}(r) , g_{n, \kappa}: = P_{\kappa, \varepsilon}(f_n) , h_{n, \kappa}: = f_n-g_{n, \kappa} = Q_{\kappa, \varepsilon}(f_n) . By (3.26) and observing that -\kappa\le g_{n,\kappa}\le \kappa and the constant function 1\in \mathscr{A} has finite Cheeger energy, we have

    -\kappa {\langle} L,1 {\rangle} = - {\langle} L,\kappa {\rangle}\le {\langle} L,g_{n, \kappa} {\rangle}\le {\langle} L, \kappa {\rangle} = \kappa {\langle} L,1 {\rangle}.

    On the other hand, since 0\le Q'_{n,\kappa}\le 1 if |r|\le 1 and |Q_{n,\kappa}'(r)|\le \delta if |r|\le \kappa/2 , we have

    |h_{n,\kappa}|\le |f_n|,\quad \mathop{\rm{ lip }}\nolimits h_{n,\kappa}\le \mathop{\rm{ lip }}\nolimits f_n,\quad \mathop{\rm{ lip }}\nolimits h_{n,\kappa}\le \delta \mathop{\rm{ lip }}\nolimits f_n\quad\text{if }|f_n| \lt \kappa/2.

    Applying Chebychev inequality

    \mathfrak m\big\{|f_n|\ge \kappa/2\big\}\le \frac {2^p\|f_n\|^p_{L^p(X, \mathfrak m)}}{\kappa^p},

    we can find n_0 (depending on \varepsilon,\kappa ) sufficiently big such that

    \int_X |h_{n, \kappa}|^p\, {\mathrm d} \mathfrak m\le \varepsilon^p/3,\quad \int_{\{f_n\ge \kappa/2\}} ( \mathop{\rm{ lip }}\nolimits f_n)^p\, {\mathrm d} \mathfrak m\le \varepsilon^p/3\quad\text{for every }n\ge n_0,

    so that

    \int_X ( \mathop{\rm{ lip }}\nolimits h_{n,\kappa})^p\, {\mathrm d} \mathfrak m \le \delta^p\int_{\{|f_n| \lt \kappa/2\}} ( \mathop{\rm{ lip }}\nolimits f_n)^p\, {\mathrm d} \mathfrak m +\int_{\{|f_n|\ge 1/2\}} ( \mathop{\rm{ lip }}\nolimits f_n)^p\, {\mathrm d} \mathfrak m \le \delta^p E^p+\frac 13 \varepsilon^p\le \frac 23 \varepsilon^p.

    By (3.8a) it follows that

    \liminf\limits_{n\to\infty}| {\langle} L,f_n {\rangle}|\le \liminf\limits_{n\to\infty}\big(| {\langle} L,g_{n, \kappa} {\rangle}|+| {\langle} L,h_{n, \kappa} {\rangle}|\big)\le \kappa {\langle} L,1 {\rangle}+D \varepsilon.

    Since \varepsilon,\kappa are arbitrary, we get \liminf_{n\to\infty}| {\langle} L,f_n {\rangle}| = 0 .

    Definition 3.14 (Measure with finite dual energy). A Radon measure \mu\in {{\mathcal M}}_+(X) has finite energy if there exists a constant D>0 such that

    \begin{equation} \int_X f\, {\mathrm d}\mu\le D\Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f)\Big)^{1/p} \quad{for\; every\; nonnegative }\;f\in \mathop{{\rm{Lip}}}\nolimits_{b}(X,\tau, {\mathsf d}). \end{equation} (3.27)

    Corollary 3.15 (Measures with finite dual energy belong to \mathscr{A}_q'( \mathbb{X}) ). If \mu\in {{\mathcal M}}_+(X) has finite energy then the linear functional f\mapsto \int_X f\, {\mathrm d}\mu on \mathscr{A}( \mathbb{X}) = \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) belongs to \mathscr{A}_q'( \mathbb{X}) and can be uniquely extended to a functional L_\mu\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) satisfying

    \begin{equation} \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_\mu) = \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*(\mu)\quad\mathit{\text{for every}}\; \kappa \gt 0. \end{equation} (3.28)

    In particular there exists a unique element u_ \kappa = {\mathrm Q}_{p, \kappa}^{-1}(L_\mu)\in H^{1,p}( \mathbb{X}) minimizing (3.14) with L = L_\mu . u_ \kappa satisfies the variational inequality

    \begin{equation} \int_X f\, {\mathrm d}\mu- \kappa\int_X {\mathrm J}_p(u_ \kappa)f\, {\mathrm d} \mathfrak m\le \frac1p \mathsf{C\kern-1.5pt E}_p(u_ \kappa+f)-\frac 1p \mathsf{C\kern-1.5pt E}(u_ \kappa)\quad \mathit{\text{for every}}\;f\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}). \end{equation} (3.29)

    Proof. We can apply Theorem 3.13 with \mathscr{A} = \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) . Clearly (3.26) holds; by decomposing f\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) as the difference f = f_+-f_- of its positive and negative part and recalling that \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f_\pm)\le \mathsf{p\kern-1pt C\kern-1.5pt E}_p(f) , (3.27) yields (3.8a) with constant 2D . (3.29) follows by (3.15) and the definition of {\mathrm A}_p given in (3.11).

    Definition 4.1 ((Nonparametric) dynamic plans). A (nonparametric) dynamic plan is a Radon measure {\boldsymbol \pi}\in \mathcal M_+( \mathrm{RA}(X)) on \mathrm{RA}(X) such that

    \begin{equation} {\boldsymbol \pi}(\ell): = \int_{ \mathrm{RA}(X)}\ell(\gamma)\, {\mathrm d} {\boldsymbol \pi}(\gamma) \lt \infty. \end{equation} (4.1)

    Using the universally Lusin-measurable map R: \mathrm{RA}(X)\to \mathrm{BVC}_c([0,1];X) in (2.7) we can also lift {\boldsymbol \pi} to a Radon measure \tilde {\boldsymbol \pi} = R_\sharp {\boldsymbol \pi} on the subset \mathrm{BVC}_c([0,1];X) of \mathrm{BVC}([0,1];X) defined in (2.6). Conversely, any Radon measure \tilde {\boldsymbol \pi} on {\mathrm C}([0,1];(X,\tau)) concentrated on \mathrm{BVC}([0,1];X) yields the Radon measure {\boldsymbol \pi}: = \mathfrak{q}_\sharp\tilde {\boldsymbol \pi} on \mathrm{RA}(X) . Notice that \mathfrak{q}_\sharp( R_\sharp {\boldsymbol \pi}) = {\boldsymbol \pi} .

    If {\boldsymbol \pi} is a dynamic plan in \mathcal M_+( \mathrm{RA}(X)) , thanks to Theorem 2.6(e) and Fubini's Theorem [11, Chap. Ⅱ-14], we can define the Borel measure \mu_ {\boldsymbol \pi}: = {\rm{Proj}} ({\boldsymbol \pi})\in \mathcal M_+(X) by the formula

    \begin{equation*} \int f\, {\mathrm d}\mu_ {\boldsymbol \pi}: = \iint_\gamma f\, {\mathrm d} {\boldsymbol \pi}(\gamma) \quad\text{for every bounded Borel function }f:X\to \mathbb{R}. \end{equation*}

    \mu_ {\boldsymbol \pi} is a Radon measure with total mass {\boldsymbol \pi}(\ell) given by (4.1) [21, §8] and it can also be considered as the integral w.r.t. {\boldsymbol \pi} of the Borel family of measures \nu_\gamma , \gamma\in \mathrm{RA}(X) [11, Chap. Ⅱ-13], in the sense that

    \int_X f\, {\mathrm d}\mu_ {\boldsymbol \pi}(x) = \int_{ \mathrm{RA}(X)}\Big(\int_X f\, {\mathrm d}\nu_\gamma\Big)\, {\mathrm d} {\boldsymbol \pi}(\gamma).

    Recall that p,q\in (1,\infty) is a fixed pair of conjugate exponents.

    Definition 4.2. We say that {\boldsymbol \pi}\in \mathcal M_+( \mathrm{RA}(X)) has barycenter in L^q(X, \mathfrak m) if there exists h\in L^q(X, \mathfrak m) such that \mu_ {\boldsymbol \pi} = h \mathfrak m , or, equivalently, if

    \int \int_\gamma f\, {\mathrm d} {\boldsymbol \pi}(\gamma) = \int fh\, {\mathrm d} \mathfrak m\quad {for \; every \;bounded\; Borel\;function }\;f:X\to \mathbb{R},

    and we call \operatorname{Bar}_{q} ( {\boldsymbol \pi}): = \|h\|_{L^q(X, \mathfrak m)} the barycentric q -entropy of {\boldsymbol \pi} . We will denote by {{\mathcal{B}}_q}\left( {{\rm{RA}}\left( X \right)} \right) the set of all plans with barycenter in L^q(X, \mathfrak m) and we will set \operatorname{Bar}_{q}( {\boldsymbol \pi}): = +\infty if {\boldsymbol \pi}\not\in {{\mathcal{B}}_q}\left( {{\rm{RA}}\left( X \right)} \right) .

    \operatorname{Bar}_{q}: \mathcal M_+( \mathrm{RA}(X))\to[0,+\infty] is a convex and positively 1 -homogeneous functional, which is lower semicontinuous w.r.t. the weak topology of \mathcal M_+( \mathrm{RA}(X)) , since it can also be characterized as the L^q entropy of the projected measure \mu_ {\boldsymbol \pi} = {\rm{Proj}} ({\boldsymbol \pi}) with respect to \mathfrak m :

    \operatorname{Bar}_{q}^q( {\boldsymbol \pi}) = \mathscr{L}_q(\mu_ {\boldsymbol \pi}| \mathfrak m),

    where for an arbitrary \sigma\in \mathcal M_+(X)

    \begin{equation} \mathscr L_q(\sigma| \mathfrak m): = \begin{cases} \int_X \Big(\frac{ {\mathrm d}\sigma}{ {\mathrm d} \mathfrak m}\Big)^q\, {\mathrm d} \mathfrak m&\text{if $\sigma\ll \mathfrak m$,}\\ +\infty&\text{otherwise.} \end{cases} \end{equation} (4.2)

    Notice that \operatorname{Bar}_{q}( {\boldsymbol \pi}) = 0 iff {\boldsymbol \pi} is concentrated on the set of constant arcs in \mathrm{RA}(X) .

    For every \mu_0,\mu_1\in \mathcal M_+(X) we will consider the (possibly empty) set

    \Pi(\mu_0,\mu_1): = \Big\{ {\boldsymbol \pi}\in \mathcal M_+( \mathrm{RA}(X)):\ ( {\mathsf e}_i)_\sharp {\boldsymbol \pi} = \mu_i\Big\}

    and we define

    \begin{equation} \mathscr{D}_q(\mu_0,\mu_1): = \inf\Big\{\operatorname{Bar}_{q}^q( {\boldsymbol \pi}): {\boldsymbol \pi}\in \Pi(\mu_0,\mu_1)\Big\}, \end{equation} (4.3)

    with the usual convention \mathscr{D}_q(\mu_0,\mu_1): = +\infty if \Pi(\mu_0,\mu_1) = \emptyset . Clearly \mathscr{D}_q(\mu_0,\mu_1) = +\infty if \mu_0(X)\neq \mu_1(X) .

    Assuming that (X, {\mathsf d}) is complete, it is possible to show ([21, §11]) that whenever \mathscr{D}_q(\mu_0,\mu_1)<+\infty the infimum in (4.3) is attained and the set of optimal plans \Pi_o(\mu_0,\mu_1) is a compact convex subset of \mathcal M_+( \mathrm{RA}(X)) . Morever, for every optimal {\boldsymbol \pi}\in \Pi_o(\mu_0,\mu_1) the measure \mu_o = {\rm{Proj}} ({\boldsymbol \pi}) is independent of {\boldsymbol \pi} .

    \mathscr{D}_q provides an important representation for the dual of the p -Cheeger energy.

    Theorem 4.3. ([21, Thm. 11.8]). For every \mu_0,\mu_1\in \mathcal M_+(X) we have

    \begin{equation} \mathscr{D}_q(\mu_0,\mu_1) = \mathsf{p\kern-1pt C\kern-1.5pt E}_p^*(\mu_0-\mu_1). \end{equation} (4.4)

    Remark 4.4. Let \mu = \mu_0-\mu_1 with \mu_i\in \mathcal M_+(X) and let \mu_+,\mu_-\in \mathcal M_+(X) be mutually singular Radon measures providing the Jordan decomposition of \mu as \mu_+-\mu_- with \mu' = \mu_0-\mu_+ = \mu_1-\mu_-\in \mathcal M_+(X) . (4.4) shows that

    \mathscr{D}_q(\mu_0,\mu_1) = \mathsf{p\kern-1pt C\kern-1.5pt E}_p^*(\mu_0-\mu_1) = \mathsf{p\kern-1pt C\kern-1.5pt E}_p^*(\mu_+-\mu_-) = \mathscr{D}_q(\mu_+,\mu_-).

    Denoting by \mathfrak c:X\to \mathrm{RA}(X) the map that at every point x associates the constant curve taking values x , if {\boldsymbol \pi}_o\in \Pi_o(\mu_+,\mu_-) and {\boldsymbol \pi}': = \mathfrak c_\sharp \mu' , it is easy to check that {\boldsymbol \pi}: = {\boldsymbol \pi}_o+ {\boldsymbol \pi}'\in \Pi_o(\mu_0,\mu_1) .

    Definition 4.5. For every nonparametric dynamic plan {\boldsymbol \pi}\in \mathcal M_+( \mathrm{RA}(X)) and \kappa>0 we define

    \mathscr{E}_{q, \kappa}( {\boldsymbol \pi}): = \operatorname{Bar}_{q}^q( {\boldsymbol \pi})+ \kappa^{-q/p} \mathscr{L}_q(( {\mathsf e}_1)_\sharp {\boldsymbol \pi}| \mathfrak m).

    For every \mu\in \mathcal M_+(X) we set

    \begin{equation} \mathscr{D}_{q, \kappa}(\mu): = \inf\Big\{ \mathscr{E}_{q, \kappa}( {\boldsymbol \pi}): {\boldsymbol \pi}\in \mathcal M_+( \mathrm{RA}(X)),\quad ( {\mathsf e}_{0})_\sharp {\boldsymbol \pi} = \mu\Big\}. \end{equation} (4.5)

    Theorem 4.6. For every \mu\in \mathcal M_+(X) we have

    \mathscr{D}_{q, \kappa}(\mu) = \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*(\mu).

    Moreover, if one of the above quantities is finite

    (a) The infimum in (4.5) is attained and there exists a unique pair of functions f_{ \kappa},g_{ \kappa}\in L^q(X, \mathfrak m) such that for every optimal plan {\boldsymbol \pi}

    g_{ \kappa} \mathfrak m = {\rm{Proj}} ({\boldsymbol \pi}),\quad \mu_ \kappa = f_{ \kappa} \mathfrak m = ( {\mathsf e}_1)_\sharp {\boldsymbol \pi},\quad {\boldsymbol \pi}\in \Pi_o(\mu,\mu_ \kappa).

    (b) There exists a unique solution u_ \kappa = {\mathrm Q}_{p, \kappa}^{-1}(L_\mu) of

    \mathrm {A}_p u+ \kappa {\mathrm J}_p u\ni L_\mu

    and it satisfies

    \begin{equation} {\mathrm J}_p\big(| {\mathrm D} u|_\star\big) = g_{ \kappa},\quad \kappa {\mathrm J}_p u = f_{ \kappa} \end{equation} (4.6)
    \left\langle L_\mu,u \right\rangle = \mathsf{C\kern-1.5pt E}_{p, \kappa}(u) = \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_\mu) = \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*(\mu).

    Moreover, setting \mu_\pm: = (\mu-\mu_ \kappa)_\pm and \mu': = \mu-\mu_+ = \mu_ \kappa-\mu_- , we can always choose {\boldsymbol \pi} = {\boldsymbol \pi}_o+ {\boldsymbol \pi}' where {\boldsymbol \pi}_o\in \Pi_o(\mu_+,\mu_-) , {\boldsymbol \pi}' = \mathfrak c_\sharp \mu' , \mathsf{p\kern-1pt C\kern-1.5pt E}_{p}^*(\mu-\mu_ \kappa) = \operatorname{Bar}_{q}^q(\pi) = \operatorname{Bar}_{q}^q(\pi_o) .

    Proof. By Corollary 3.15 we can extend \mu to a functional L_\mu\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) satisfying (3.28). We can then apply Proposition 3.9 and find nonnegative f_ \kappa\in L^q(X, \mathfrak m) and u_ \kappa\in L^p(X, \mathfrak m) such that

    \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_\mu) = \mathsf{C\kern-1.5pt E}_{p}^*(L_\mu-f_ \kappa)+ \kappa^{-q/p}\|f_ \kappa\|_{L^q(X, \mathfrak m)}^q = \mathsf{p\kern-1pt C\kern-1.5pt E}_{p}^*(\mu-f_ \kappa \mathfrak m)+ \kappa^{-q/p} \mathscr{L}_q(f_ \kappa \mathfrak m| \mathfrak m)

    and u_\kappa satisfies f_ \kappa = \kappa {\mathrm J}_p u_ \kappa and (3.15) with L = L_\mu . Setting \mu_ \kappa: = f_ \kappa \mathfrak m and selecting {\boldsymbol \pi}\in \Pi_o(\mu,\mu_ \kappa) according to Remark 4.4, (4.4) yields

    \mathsf{C\kern-1.5pt E}_{p, \kappa}^*(L_\mu) = \operatorname{Bar}_{q}^q( {\boldsymbol \pi})+ \kappa^{-q/p} \mathscr{L}_q(( {\mathsf e}_1)_\sharp {\boldsymbol \pi}| \mathfrak m) = \mathscr{E}_{q, \kappa}( {\boldsymbol \pi})\ge \mathscr{D}_{q, \kappa}(\mu).

    On the other hand, it is easy to check that \mathscr{D}_{q, \kappa}(\mu)\ge \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}^*(\mu) , since for every plan {\boldsymbol \pi}\in {{\mathcal M}}_+( \mathrm{RA}(X)) as in (4.5) and every f\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d})

    \begin{align*} \int_X f\, {\mathrm d} \mu &\le \int_X f\, {\mathrm d}( {\mathsf e}_0)_\sharp {\boldsymbol \pi} - \int_X f\, {\mathrm d}( {\mathsf e}_1)_\sharp {\boldsymbol \pi}+ \int_X f\, {\mathrm d}( {\mathsf e}_1)_\sharp {\boldsymbol \pi} \\&\le \int_{ \mathrm{RA}(X)}\int_\gamma \mathop{{\rm{lip}}}\nolimits f\, {\mathrm d} {\boldsymbol \pi}(\gamma)+ \|f\|_{L^p}\Big( \mathscr{L}_q(( {\mathsf e}_1)_\sharp {\boldsymbol \pi}| \mathfrak m)\Big)^{1/q} \\&\le \big\| \mathop{{\rm{lip}}}\nolimits f\big\|_{L^p}\operatorname{Bar}_{q}( {\boldsymbol \pi})+ \|f\|_{L^p}\Big( \mathscr{L}_q(( {\mathsf e}_1)_\sharp {\boldsymbol \pi}| \mathfrak m)\Big)^{1/q} \le \Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p, \kappa}(f)\Big)^{1/p}\,\Big( \mathscr{E}_{q, \kappa}( {\boldsymbol \pi})\Big)^{1/q}. \end{align*}

    Using now the fact that {\langle} L_\mu-f_ \kappa,u {\rangle} = \operatorname{Bar}_{q}^q( {\boldsymbol \pi}) = \mathsf{C\kern-1.5pt E}_p(u_ \kappa) we get

    \begin{equation} \int_X g_ \kappa^q\, {\mathrm d} \mathfrak m = \int_X | {\mathrm D} u_ \kappa|_\star^p\, {\mathrm d} \mathfrak m = {\langle} L_\mu-f_ \kappa,u_ \kappa {\rangle}. \end{equation} (4.7)

    We can also select a sequence w_n\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) such that w_n\to u_ \kappa , \mathop{{\rm{lip}}}\nolimits w_n \to | {\mathrm D} u_ \kappa|_\star strongly in L^p(X, \mathfrak m) , so that

    \begin{align*} {\langle} L_\mu-f_ \kappa,u_ \kappa {\rangle} & = \lim\limits_{n\to\infty} {\langle} L_\mu-f_ \kappa,w_n {\rangle} = \lim\limits_{n\to\infty} \Big(\int_X w_n\, {\mathrm d}\mu-\int_X w_n\, {\mathrm d}\mu_1\Big) \\& = \lim\limits_{n\to\infty} \Big(\int_{ \mathrm{RA}(X)} w_n( {\mathsf e}_0(\gamma))-w_n( {\mathsf e}_1(\gamma))\Big)\, {\mathrm d} {\boldsymbol \pi}(\gamma) \\&\le \limsup\limits_{n\to\infty} \int_{ \mathrm{RA}(X)} \int_\gamma \mathop{{\rm{lip}}}\nolimits w_n \, {\mathrm d} {\boldsymbol \pi}(\gamma) = \limsup\limits_{n\to\infty} \int_X g_ \kappa \mathop{{\rm{lip}}}\nolimits w_n\, {\mathrm d} \mathfrak m = \int_X g_ \kappa| {\mathrm D} u|_\star\, {\mathrm d} \mathfrak m. \end{align*}

    Inserting this inquality in (4.7) we obtain the first identity of (4.6).

    Let us give a first important application of the above result to the representation of positive functionals.

    Theorem 4.7. Let \mathscr{A} be a compatible subalgebra of \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) and let L be functional on \mathscr{A} satisfying

    | {\langle} L,f {\rangle}|\le D\Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{1,p}(f)\Big)^{1/p}\quad{for \; every\; f \in \mathscr{A} ,}\quad {\langle} L, f {\rangle}\ge0\quad {for\; every\; positive }\;f\in \mathscr{A}.

    Then L admits a unique extension \tilde L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) and there exists a unique \mu\in \mathcal M_+(X) representing L as

    \begin{equation} {\langle} L,f {\rangle} = \int_X f\, {\mathrm d}\mu\quad\mathit{\text{for every}}\;f\in \mathscr{A}. \end{equation} (4.8)

    Proof. By Theorem 3.13 and Proposition 3.7(c) we know that L is the restriction to \mathscr{A} of a unique functional \tilde L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) . It is easy to check that \tilde L is also positive on H^{1,p}( \mathbb{X}) and applying Proposition 3.9 we can find a sequence w_n\in L^q(X, \mathfrak m) , w_n\ge0 , strongly converging to \tilde L in H^{-1,q}( \mathbb{X}) . Let \mu_n: = w_n \mathfrak m and \nu_n: = {\mathrm R}_1(w_n) \mathfrak m ; applying Theorem 4.6 we can find optimal dynamic plans {\boldsymbol \pi}_n\in \Pi_o(\mu_n,\nu_n) such that \operatorname{Bar}_{q}( {\boldsymbol \pi}_n) = \mathsf{p\kern-1pt C\kern-1.5pt E}_{1,p}^*(\mu_n-\nu_n)\le C . Since \mathscr{L}_q(\nu_n| \mathfrak m) is also uniformly bounded, the sequence {\boldsymbol \pi}_n satisfies the tightness criterium of [21, Lemma 8.5], so that it admits a subsequence (still denoted by {\boldsymbol \pi}_n ) weakly converging to {\boldsymbol \pi}\in {{\cal B}_q}\left( {{\rm{RA}}\left( X \right)} \right) \subset \mathcal M_+( \mathrm{RA}(X)) .

    The Radon measure \mu: = ( {\mathsf e}_0)_\sharp {\boldsymbol \pi} is the weak limit of \mu_n : in particular, for every f\in \mathscr{A}

    \left\langle L,f \right\rangle = \left\langle \tilde L,f \right\rangle = \lim\limits_{n\to\infty}\int_X f\, {\mathrm d}\mu_n = \int_X f\, {\mathrm d}\mu.

    In this last section we apply the previous result to prove new properties of the Newtonian capacity. We first recall the basic facts about the Newtonian approach [18,23], based on the notion of p -Modulus which has been introduced by Fuglede [15] in the natural framework of collection of positive measures, as in [1]. We refer to [8,17] for a comprehensive presentation of this topic. As usual, p,q\in (1,\infty) denote a pair of conjugate exponents.

    Definition 5.1 ( p -Modulus of a family of rectifiable arcs). The p -Modulus of a collection \Gamma\subset \mathrm{RA}(X) is defined by

    \operatorname{Mod}_{p}(\Gamma) : = \inf \left\{ \int_X f^p \, {\mathrm d} \mathfrak m: \, f:X\to[0,\infty] \ \text{is Borel},\ \int_\gamma f \geq 1 \; \; { for \;all }\; \gamma \in \Gamma \right\}.

    \Gamma is said to be \operatorname{Mod}_{p} -negligible if \operatorname{Mod}_{p}(\Gamma) = 0 . We say that a property P on \mathrm{RA}(X) holds \operatorname{Mod}_{p} -a.e. if the set of arcs where P fails is \operatorname{Mod}_{p} -negligible. We say that a property P on X holds quasi everywhere (q.e.) if the set of points E where P fails is \mathfrak m -negligible and satisfies

    \operatorname{Mod}_{p}(\Gamma_E) = 0\quad {where} \quad \Gamma_E: = \Big\{\gamma\in \mathrm{RA}(X): \ell(\gamma) \gt 0,\ {\mathsf e}[\gamma]\cap E \ne \emptyset\Big\}.

    Notice that if E is \mathfrak m -negligible then for \operatorname{Mod}_{p} -a.e. arc \gamma the set \{t\in [0,1]: R_\gamma(t)\in E\} is {\mathscr L}^{1} -negligible. It can be shown (see e.g., [8]) that \operatorname{Mod}_{p} is an increasing and subadditive functional which is continuous along increasing sequences. In fact, by [1] and [21, §7.2], \operatorname{Mod}_{p} is also continuous along decreasing sequence of compact sets, so that it is a Choquet capacity for the compact paving in \mathrm{RA}(X) [11, Chap. Ⅲ, 28].

    It is not difficult to check that for every dynamic plan {\boldsymbol \pi}\in {{\cal B}_q}\left( {{\rm{RA}}\left( X \right)} \right) and every {\boldsymbol \pi} -measurable set \Gamma\subset \mathrm{RA}(X)

    {\boldsymbol \pi}(\Gamma)\le \operatorname{Bar}_{q}( {\boldsymbol \pi}) \operatorname{Mod}_{p}^{1/p}(\Gamma),

    which in particular shows that Borel \operatorname{Mod}_{p} -negligible sets are also {\boldsymbol \pi} -negligible for every {\boldsymbol \pi}\in {{\cal B}_q}\left( {{\rm{RA}}\left( X \right)} \right) . In fact, a much more refined result holds [1,21]:

    Theorem 5.2. If \mathbb{X} is a complete e.m.t.m. space and \tau is a Souslin topology for X , then every Borel or Souslin set \Gamma in \mathrm{RA}(X) is \operatorname{Mod}_{p} -capacitable and satisfies

    \Big( \operatorname{Mod}_{p}(\Gamma)\Big)^{1/p} = \sup\Big\{ {\boldsymbol \pi}(\Gamma): {\boldsymbol \pi}\in \mathcal M_+( \mathrm{RA}(X)),\ \operatorname{Bar}_{q}( {\boldsymbol \pi})\le 1\Big\}.

    In particular, \Gamma is \operatorname{Mod}_{p} -negligible if and only if {\boldsymbol \pi}(\Gamma) = 0 for every {\boldsymbol \pi}\in {{\cal B}_q}\left( {{\rm{RA}}\left( X \right)} \right) .

    Recall that {\mathsf e}_i(\gamma) , i = 0,1 , denote the initial and final points of a rectifiable arc \gamma\in \mathrm{RA}(X) .

    Definition 5.3 (Newtonian weak upper gradient). Let f:X\to \mathbb{R} be \mathfrak m -measurable and p -summable function. We say that f belongs to the Newtonian space N^{1,p}( \mathbb{X}) if there exists a nonnegative g\in L^p(X, \mathfrak m) such that

    \begin{equation} \Big|f( {\mathsf e}_1(\gamma))-f( {\mathsf e}_0(\gamma))\Big|\le \int_\gamma g\quad{for \; \operatorname{Mod}_{p}-a.e.\; arc \; \gamma\in \mathrm{RA}(X)}. \end{equation} (5.1)

    In this case, we say that g is a N^{1,p} -weak upper gradient of f .

    Functions with \operatorname{Mod}_{p} -weak upper gradient have the important Beppo-Levi property of being absolutely continuous along \operatorname{Mod}_{p} -a.e. arc \gamma (more precisely, this means f\circ R_\gamma is absolutely continuous, see [23, Proposition 3.1]). Notice that functions in N^{1,p}( \mathbb{X}) are everywhere defined. We say that \tilde f\in N^{1,p}( \mathbb{X}) is a good representative of a function f\in L^p(X, \mathfrak m) if \mathfrak m(\{\tilde f\neq f\}) = 0 .

    Weak upper gradient enjoys a strong stability property [8, Prop. 2.3], resulting from a refined version of Fuglede's Lemma:

    Theorem 5.4. Let f_n\in N^{1,p}( \mathbb{X}), g_n\in L^p(X, \mathfrak m) be two sequences strongly converging to f,g\in L^p(X, \mathfrak m) respectively in L^p(X, \mathfrak m) . If g_n is a N^{1,p} -weak upper gradient of f then there exists a good representative \tilde f\in N^{1,p}( \mathbb{X}) of f and a subsequence k\mapsto n(k) such that f_{n(k)}\to \tilde f quasi everywhere; moreover g is a N^{1,p} -weak upper gradient of \tilde f .

    It is clear that a function f\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) belongs to N^{1,p}( \mathbb{X}) and \mathop{{\rm{lip}}}\nolimits f is a N^{1,p} -weak upper gradient (it is in fact an upper gradient). By Theorems 5.4 and 3.3 one can easily get that also every f\in H^{1,p}( \mathbb{X}) admits a good representative \tilde f\in N^{1,p}( \mathbb{X}) and | {\mathrm D} f|_\star is a N^{1,p} -weak upper gradient of \tilde f . Equivalently, \tilde f is absolutely continuous along \operatorname{Mod}_{p} -a.e. arc and g satisfies (5.1) \operatorname{Mod}_{p} -a.e.

    In fact these two notions are essentially equivalent modulo the choice of a representative in the equivalence class [1,5,6,21]:

    Theorem 5.5. Let us suppose that \mathbb{X} is a complete e.m.t.m. space. Every function f\in N^{1,p}( \mathbb{X}) also belongs to H^{1,p}( \mathbb{X}) and every N^{1,p} -weak upper gradient g satisfies g\ge | {\mathrm D} f|_\star \mathfrak m -a.e., so that | {\mathrm D} f|_\star is also the minimal N^{1,p} -weak upper gradient of f .

    The Newtonian capacity \operatorname{Cap}_p(E) of a subset E\subset X can be defined as

    \begin{equation} \operatorname{Cap}_p(E): = \inf\Big\{ \mathsf{C\kern-1.5pt E}_{p,1}(u):u\in N^{1,p}( \mathbb{X}),\ u\ge 1\text{ on $E$}\Big\}. \end{equation} (5.2)

    By choosing u as the function taking the constant value 1 it is immediate to check that in our setting the capacity of a set is always finite and

    \operatorname{Cap}_p(E)\le \mathfrak m(X) \lt \infty\quad\text{for every }E\subset X.

    It can be proved [8, Prop. 1.48] that

    \begin{equation} \text{$E\subset X$ has $0$ capacity if and only if $E$ is $ \mathfrak m$-negligible and } \operatorname{Mod}_{p}(\Gamma_E) = 0, \end{equation} (5.3)

    so that a property P on X holds quasi everywhere if the set where P fails has 0 capacity. Notice that if \tilde f_i\in N^{1,p}( \mathbb{X}) coincide \mathfrak m -a.e., then \tilde f_1 = \tilde f_2 q.e. in X . Notice moreover that we can use q.e. inequality in (5.2), i.e.,

    u\in N^{1,p}( \mathbb{X}),\quad u\ge 1 \text{ q.e. on }E\quad\Rightarrow\quad \operatorname{Cap}_p(E)\le \mathsf{C\kern-1.5pt E}_{p,1}(u).

    We also recall that the capacity satisfies the following properties [8, Thm 1.27, Prop. 2.22, Thm. 6.4]:

    \begin{align*} & \operatorname{Cap}_p(\emptyset) = 0 \\& \mathfrak m(E)\le \operatorname{Cap}_p(E) \\& E_1\subset E_2\quad\Rightarrow\quad \operatorname{Cap}_p(E_1)\le \operatorname{Cap}_p(E_2) \\& \operatorname{Cap}_p(E_1\cup E_2)+ \operatorname{Cap}_p(E_1\cap E_2)\le \operatorname{Cap}_p(E_1)+ \operatorname{Cap}_p(E_2) \\& \operatorname{Cap}_p\Big(\bigcup_{n = 1}^\infty E_n)\le \sum\limits_{n = 1}^\infty \operatorname{Cap}_p(E_n) \\& E_n\uparrow E\quad\Rightarrow\quad \operatorname{Cap}_p(E) = \lim\limits_{n\to\infty} \operatorname{Cap}_p(E_n) = \sup\limits_{n \gt 0} \operatorname{Cap}_p(E_n). \end{align*}

    We want now to study the relation between the Newtonian capacity and measures \mu\in {{\mathcal M}}_+(X) with finite energy, according to Definition 3.14. We will denote by

    \begin{equation} \mu = \mu^a+\mu^\perp,\quad \mu^a = \varrho \mathfrak m\ll \mathfrak m,\quad \mu^\perp\perp \mathfrak m \end{equation} (5.4)

    the canonical Lebesgue decomposition of \mu w.r.t. \mathfrak m . Since by a simple truncation argument it is easy to check that

    \frac 1q \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu) = \sup\Big\{\int_X f\, {\mathrm d}\mu-\frac1p \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}(f):f\in \mathop{\rm{Lip }}\nolimits_b(X,\tau, {\mathsf d}),\ f\ge0\Big\}

    we obtain that

    \mu\le \nu\quad\Rightarrow\quad \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)\le \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\nu).

    In particular \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu^\perp)\le \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)<\infty .

    Theorem 5.6. Let \mu\in {{\mathcal M}}_+(X) be a measure with finite energy and let L_\mu\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) be the linear functional associated to \mu according to Corollary 3.15.

    (a) If E is a universally measurable subset of X with 0 capacity then E is \mu -negligible.

    (b) If u\in H^{1,p}( \mathbb{X}) is nonnegative and \tilde u\in N^{1,p}( \mathbb{X}) is a good representative of u , then \tilde u\in L^1(X,\mu) and

    \begin{equation} {\langle} L_\mu,u {\rangle} = \int_X \tilde u\, {\mathrm d}\mu\le \Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)\Big)^{1/q} \Big( \mathsf{C\kern-1.5pt E}_{p,1}(u)\Big)^{1/p}. \end{equation} (5.5)

    Proof. (a) Let E be a set with 0 capacity according to (5.3); since \mathfrak m(E) = 0 , by considering the Lebesgue decomposition (5.4) it is sufficient to show that \mu^\perp(E) = 0 . It is not restrictive to assume \mu^\perp(X)>0 ; by Theorem 4.6 we can find a plan {\boldsymbol \pi}\in {{\cal B}_q}\left( {{\rm{RA}}\left( X \right)} \right) such that

    \mu^\perp = ( {\mathsf e}_0)_\sharp {\boldsymbol \pi}.

    It follows that

    \mu^\perp(E) = {\boldsymbol \pi}\big\{\gamma\in \mathrm{RA}(X): {\mathsf e}_0(\gamma)\in E\big\} \le {\boldsymbol \pi}(\Gamma_E)\le \operatorname{Mod}_{p}(\Gamma_E)\operatorname{Bar}_{q} ( {\boldsymbol \pi}) = 0.

    (b) Let us first assume that \tilde u\le M for some constant M>0 . We can find a sequence u_n\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) taking values in [0,M] , converging to \tilde u q.e. and with u_n\to u, \mathop{{\rm{lip}}}\nolimits u_n\to | {\mathrm D} u|_\star in L^2(X, \mathfrak m) . The uniform bound, the q.e. convergence and the fact that L_\mu\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) yield

    \lim\limits_{n\to\infty} \int_X u_n\, {\mathrm d}\mu = \int_X \tilde u\, {\mathrm d}\mu = {\langle} L_\mu,\tilde u {\rangle} = {\langle} L_\mu,u {\rangle}\le \Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)\Big)^{1/q} \Big( \mathsf{C\kern-1.5pt E}_{p,1}(u)\Big)^{1/p}

    The case of a general nonnegative u follows by passing to the limit in the sequence u_M: = u\land M as M \uparrow\infty and using monotone convergence.

    Theorem 5.7. For every Borel set E\subset X and every measure \mu\in {{\mathcal M}}_+(X) with finite energy we have

    \begin{equation} \mu(E)\le \Big( \operatorname{Cap}_p(E)\Big)^{1/p}\Big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)\Big)^{1/q}. \end{equation} (5.6)

    If F\subset X is a closed set then there exists \mu\in {{\mathcal M}}_+(X) supported in F with

    \mu(F) = \operatorname{Cap}_p(F) = \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu) = \mathsf{C\kern-1.5pt E}_{p,1}(u)

    where u\in \mathbb{N}^{1,p}( \mathbb{X}) realizes the infimum of (5.2) and

    L_\mu = {\mathrm J}_pu+ {\mathrm A}_p u\quad\text{in } H^{-1,q}_{\mathrm{pd}}(\mathbb{X}).

    Equivalently, for every closed set F\subset X

    \begin{equation} \big( \operatorname{Cap}_p(F)\big)^{1/p} = \max \Big\{\mu(F):\mu\in {{\mathcal M}}_+(X),\ \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)\le 1\Big\}. \end{equation} (5.7)

    Proof. (5.6) follows easily by (5.5).

    Let us now consider the case when F is closed and let us set {{\mathcal K}}: = \{u\in N^{1,2}( \mathbb{X}):\ u\ge 1\; \text{ q.e. on }\;F\} ; {{\mathcal K}} can be identified with a convex subset of H^{1,p}( \mathbb{X}) . Let u_n\in {{\mathcal K}} be a sequence satisfying \lim\limits_{n\to\infty} \mathsf{C\kern-1.5pt E}_{p,1}(u_n) = \operatorname{Cap}_p(F) . By a truncation argument, it is not restrictive to assume that 0\le u_n\le 1 .

    By applying Mazur's Theorem and Theorem 5.4 it is not restrictive to assume that there exists u\in N^{1,p}( \mathbb{X}) such that

    u_n\to u\quad\text{q.e.},\quad \big\|u_n-u\big\|_{L^2}+\big\| | {\mathrm D} u_n|_\star-| {\mathrm D} u|_\star\big\|_{L^2}\to 0

    Up to modifying u in a set of 0 capacity, we can thus suppose that u\in {{\mathcal K}} , 0\le u\le 1 , and \mathsf{C\kern-1.5pt E}_{p,1}(u) = \operatorname{Cap}_p(F) . The minimality yields that there exists L\in {\mathrm A}_p(u)+ {\mathrm J}_p(u)\subset H^{-1,p}( \mathbb{X}) such that

    {\langle} L,v-u {\rangle}\ge0\quad \text{for every }v\in {{\mathcal K}}.

    In particular, choosing v = u+\phi with \phi nonnegative we get

    {\langle} L,\phi {\rangle}\ge0\quad \text{for every $ \phi\in N^{1,p}( \mathbb{X}) $, $ \phi\ge0 $},

    so that L\in H^{-1,q}_{\mathrm{pd}}(\mathbb{X}) and its action on bounded Lipschitz functions can be represented by a positive Radon measure \mu according to (4.8) thanks to Theorem 4.7

    Choosing now \phi\in \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) such that \phi\equiv0 on F we get

    {\langle} L,\phi {\rangle} = \int_X \phi\, {\mathrm d}\mu = 0,

    so that \mu(X\setminus F) = 0 and \mu is concentrated on F (recall that \mathop{{\rm{Lip}}}\nolimits_b(X,\tau, {\mathsf d}) generates the \tau topology of X ). Since L has finite energy \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu) = \mathsf{C\kern-1.5pt E}_{p,1}^*( L) . Since u\in N^{1,p}( \mathbb{X}) is nonnegative it follows that

    \operatorname{Cap}_p(F) = \mathsf{C\kern-1.5pt E}_{p,1}(u) = \mathsf{C\kern-1.5pt E}_{p,1}^*( L) = {\langle} L,u {\rangle} = \int_X u\, {\mathrm d}\mu = \mu(F).

    The renormalization \tilde\mu: = \mu \big( \mathsf{p\kern-1pt C\kern-1.5pt E}_{p,1}^*(\mu)\big)^{-1/q} provides (5.7).

    The authors acknowledge support from the MIUR - PRIN project 2017TEXA3H "Gradient flows, Optimal Transport and Metric Measure Structures".

    The authors declare no conflict of interest.



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