This research paper investigated fixed point results for almost (ζ−θρ)-contractions in the context of quasi-metric spaces. The study focused on a specific class of (ζ−θρ)-contractions, which exhibit a more relaxed form of contractive property than classical contractions. The research not only established the existence of fixed points under the almost (ζ−θρ)-contraction framework but also provided sufficient conditions for the convergence of fixed point sequences. The proposed theorems and proofs contributed to the advancement of the theory of fixed points in quasi-metric spaces, shedding light on the intricate interplay between contraction-type mappings and the underlying space's quasi-metric structure. Furthermore, an application of these results was presented, highlighting the practical significance of the established theory. The application demonstrated how the theory of almost (ζ−θρ)-contractions in quasi-metric spaces can be utilized to solve real-world problems.
Citation: Gonca Durmaz Güngör, Ishak Altun. Fixed point results for almost (ζ−θρ)-contractions on quasi metric spaces and an application[J]. AIMS Mathematics, 2024, 9(1): 763-774. doi: 10.3934/math.2024039
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This research paper investigated fixed point results for almost (ζ−θρ)-contractions in the context of quasi-metric spaces. The study focused on a specific class of (ζ−θρ)-contractions, which exhibit a more relaxed form of contractive property than classical contractions. The research not only established the existence of fixed points under the almost (ζ−θρ)-contraction framework but also provided sufficient conditions for the convergence of fixed point sequences. The proposed theorems and proofs contributed to the advancement of the theory of fixed points in quasi-metric spaces, shedding light on the intricate interplay between contraction-type mappings and the underlying space's quasi-metric structure. Furthermore, an application of these results was presented, highlighting the practical significance of the established theory. The application demonstrated how the theory of almost (ζ−θρ)-contractions in quasi-metric spaces can be utilized to solve real-world problems.
Dedicated to the memory of Professor Ireneo Peral, with high feelings of admiration for his notable contributions in Mathematics and great affection
In recent years, geometric analysis and partial differential equations on the Heisenberg group have attracted great attention. In this article, we investigate some concentration–compactness results related to the Hardy–Sobolev embedding on the classical and fractional Folland–Stein spaces in the Heisenberg group. Before stating the main results, let us recall some relevant contributions in the topic.
The Heisenberg group Hn is the Lie group which has R2n+1 as a background manifold and whose group structure is given by the non–Abelian law
ξ∘ξ′=(z+z′,t+t′+2n∑i=1(yix′i−xiy′i)) |
for all ξ, ξ′∈Hn, with
ξ=(z,t)=(x1,…,xn,y1,…,yn,t) and ξ′=(z′,t′)=(x′1,…,x′n,y′1,…,y′n,t′). |
We denote by r the Korányi norm, defined as
r(ξ)=r(z,t)=(|z|4+t2)1/4, |
with ξ=(z,t), z=(x,y)∈Rn×Rn, t∈R, and |z| the Euclidean norm in R2n.
A key result, whose importance is also due to its connection with the CR Yamabe problem, is the subelliptic Sobolev embedding theorem in Hn, which is due to Folland and Stein [14]. This result is valid in the more general context of Carnot groups, but we state it in the set up of the Heisenberg group. If 1<p<Q, where Q=2n+2 is the homogeneous dimension of the Heisenberg group Hn, we know by [14] that there exists a positive constant C=C(p,Q) such that
∫Hn|φ|p∗dξ≤C∫Hn|DHφ|pHdξfor allφ∈C∞c(Hn),p∗=pQQ−p, | (1.1) |
and p∗ is the critical exponent related to p. Moreover, the vector
DHu=(X1u,⋯,Xnu,Y1u,⋯,Ynu) |
is the horizontal gradient of a regular function u, where {Xj,Yj}nj=1 is the basis of horizontal left invariant vector fields on Hn, that is
Xj=∂∂xj+2yj∂∂t,Yj=∂∂yj−2xj∂∂t,j=1,…,n. | (1.2) |
Unlike the Euclidean case, cf. [34] and [2], the value of the best constant in (1.1) is unknown. In the particular case p=2, the problem of the determination of the best constant in (1.1) is related to the CR Yamabe problem and it has been solved by the works of Jerison and Lee [21,22,23,24]. In the general case, existence of extremal functions of (1.1) was proved by Vassilev in [35] via the concentration–compactness method of Lions, see also [20]. This method does not allow an explicit determination of the best constant Cp∗ of (1.1). However, we know from [35] that Cp∗ is achieved in the Folland–Stein space S1,p(Hn), which is defined, for 1<p<Q, as the completion of C∞c(Hn) with respect to the norm
‖DHu‖p=(∫Hn|DHu|pHdξ)1/p. |
Thus, we can write the best constant Cp∗ of the Folland–Stein inequality (1.1) as
Cp∗=infu∈S1,p(Hn)u≠0‖DHu‖pp‖u‖pp∗. | (1.3) |
Note that the Euler–Lagrange equation of the nonnegative extremals of (1.1) leads to the critical equation
−ΔH,pu=|u|p∗−2u in Hn, |
where the operator ΔH,p is the well known p Kohn–Spencer Laplacian, which is defined as
ΔH,pφ=divH(|DHφ|p−2HDHφ), |
for all φ∈C2(Hn).
The study of critical equations is deeply connected to the concentration phenomena, which occur when considering sequences of approximated solutions. Indeed, given a weakly convergent sequence (uk)k in S1,p(Hn), we can infer that (uk)k is bounded in Lp∗(Hn), but we do not have compactness properties in general. On the other hand, we know that the sequences μk=|DHu|pHdξ and νk=|uk|p∗dξ weak∗ converge to some measures μ and ν in the dual space M(Hn) of all real valued, finite, signed Radon measures on Hn. An essential step in the concentration–compactness method is the study of the exact behavior of the limit measures in the space M(Hn) and in the spirit of Lions. In particular, following [25,26], Ivanov and Vassilev in [20] proved the following result.
Theorem A (Lemma 1.4.5, Ivanov and Vassilev [20]). Let (uk)k be a sequence in S1,p(Hn) such that uk⇀u in S1,p(Hn) and |uk|p∗dξ∗⇀ν, |DHuk|pHdξ∗⇀μ in M(Hn), for some appropriate u∈S1,p(Hn), and finite nonnegative Radon measures μ, ν on Hn.
Then, there exist an at most countable set J, a family of points {ξj}j∈J⊂Hn and two families of nonnegative numbers {μj}j∈J and {νj}j∈J such that
ν=|u|p∗dξ+∑j∈Jνjδξj,μ≥|DHu|pHdξ+∑j∈Jμjδξjνp/p∗j≤μjCp∗forallj∈J, |
where δξj are the Dirac functions at the points ξj of Hn.
The aim of this paper is to extend Theorem A in two different ways. First, we want to prove a version of Theorem A suitable to deal with a combined Hardy and Sobolev embedding. Indeed, following [16], we set
ψ(ξ)=|DHr(ξ)|H=|z|r(ξ) for ξ=(z,t)≠(0,0). |
Assume from now on that 1<p<Q and let φ∈C∞c(Hn∖{O}). Then, the Hardy inequality in the Heisenberg group states as follows
∫Hn|φ|pψpdξrp≤(pQ−p)p∫Hn|DHφ|pHdξ. | (1.4) |
Inequality (1.4) was obtained by Garofalo and Lanconelli in [16] when p=2 and then extended to all p>1 in [7,29]. When p=2, the optimality of the constant (2/(Q−2))2 is shown in [18]. Let us also mention that a sharp inequality of type (1.4) has been derived in general Carnot–Carathéodory spaces by Danielli, Garofalo and Phuc in [8].
Obviously, inequality (1.4) remains valid in S1,p(Hn). Moreover, inequalities (1.1) and (1.4) imply that for any σ∈(−∞,Hp) the following best constant is well defined
Iσ=infu∈S1,p(Hn)u≠0‖DHu‖pp−σ‖u‖pHp‖u‖pp∗, | (1.5) |
where
Hp=infu∈S1,p(Hn)u≠0‖DHu‖pp‖u‖pHp,‖u‖pHp=∫Hn|u|pψprpdξ. | (1.6) |
Note that, when σ=0, we recover the Sobolev embedding, that is I0=Cp∗. However, the Hardy embedding S1,p(Hn)↪Lp(Hn,ψpr−pdξ) is continuous, but not compact, even locally in any neighborhood of O, where O=(0,0) denotes the origin of Hn. A challenging problem is then to provide sufficient conditions for the existence of a nontrivial solution to critical equations with Hardy terms in the whole space Hn, when a triple loss of compactness takes place. To overcome this difficulty, we prove in Theorem 1.1 and Theorem 1.2 some versions of the concentration–compactness principle for related to the embedding (1.5).
Theorem 1.1. Let σ∈(−∞,Hp) and let (uk)k be a sequence in S1,p(Hn) such that uk⇀u in S1,p(Hn), and|uk|p∗dξ∗⇀ν, |DHuk|pHdξ∗⇀μ, |uk|pψpdξr(ξ)p∗⇀ω in M(Hn), for some appropriateu∈S1,p(Hn), and finite nonnegative Radon measures μ, ν, ω on Hn.
Then, there exist an at most countable set J, a family of points {ξj}j∈J⊂Hn, two families of nonnegative numbers {μj}j∈J and {νj}j∈J and three nonnegative numbers ν0,μ0,ω0, such that
ν=|u|p∗dξ+ν0δO+∑j∈Jνjδξj, | (1.7) |
μ≥|DHu|pHdξ+μ0δO+∑j∈Jμjδξj, | (1.8) |
ω=|u|pψpdξr(ξ)p+ω0δO, | (1.9) |
νp/p∗j≤μjCp∗forallj∈J,νp/p∗0≤μ0−σω0Iσ, | (1.10) |
whereCp∗=I0 and Iσ are defined in (1.3) and (1.5), while δO,δξj are the Dirac functions at the points O and ξj of Hn, respectively.
Theorem 1.1 extends Theorem A and also Theorem 1.2 of [5] to the case of unbounded domains, see also [20,31,32]. The strategy is the same as the one in the seminal papers of Lions [25,26], but there are some complications due to the non Euclidean context.
The whole Heisenberg group is endowed with noncompact families of dilations and translations, which could provide a loss of compactness due to the drifting towards infinity of the mass, or – in other words – the concentration at infinity. In order to deal with this type of phenomena, we prove a variant of the concentration–compactness principle of Lions, that is the concentration–compactness principle at infinity. This variant was introduced by Bianchi, Chabrowski and Szulkin in [3,6] and we prove an extension of their results suitable to deal with critical Hardy equations in the Heisenberg group.
Denote by BR(ξ) the Korányi open ball of radius R centered at ξ. For simplicity BR is the ball of radius R centered at ξ=O.
Theorem 1.2. Let (uk)k be a sequence in S1,p(Hn) as in Theorem 1.1 and define
ν∞=limR→∞lim supk→∞∫BcR|uk|p∗dξ,μ∞=limR→∞lim supk→∞∫BcR|DHuk|pHdξ, | (1.11) |
ω∞=limR→∞lim supk→∞∫BcR|uk|pψpdξr(ξ)p. | (1.12) |
Then,
lim supk→∞∫Hn|uk|p∗dξ=ν(Hn)+ν∞,lim supk→∞∫Hn|DHuk|pHdξ=μ(Hn)+μ∞, | (1.13) |
lim supk→∞∫Hn|uk|pψpdξr(ξ)p=ω(Hn)+ω∞,νp/p∗∞≤μ∞−σω∞Iσ, | (1.14) |
where μ,ν,ω are the measures introduced in Theorem 1.1.
In the second part of the paper, we want to extend the previous results to the fractional case. Let 0<s<1 and 1<p<∞. We define the fractional Sobolev space HWs,p(Hn) as the completion of C∞c(Hn) with respect to the norm
‖⋅‖HWs,p(Hn)=‖⋅‖Lp(Hn)+[⋅]H,s,p, |
where
[φ]H,s,p=(∫∫Hn×Hn|φ(ξ)−φ(η)|pr(η−1∘ξ)Q+spdξdη)1/palong anyφ∈C∞c(Hn). | (1.15) |
The fractional Sobolev embedding in the Heinseberg group was obtained in [1] following the lines of [9] and states as follows. If sp<Q, then there exists a constant Cp∗s depending on p,Q and s such that
‖φ‖pp∗s≤Cp∗s[φ]pH,s,pfor allφ∈C∞c(Hn),p∗s=pQQ−sp. | (1.16) |
The proof of the above inequality is obtained directly, by extending the method of [9] to the Heisenberg context.
For notational simplicity, the fractional (s,p) horizontal gradient of any function u∈HWs,p(Hn) is denoted by
|DsHu|p(ξ)=∫Hn|u(ξ)−u(η)|pr(η−1∘ξ)Q+psdη=∫Hn|u(ξ∘h)−u(ξ)|pr(h)Q+psdh. | (1.17) |
Note that the (s,p) horizontal gradient of a function u∈HWs,p(Hn) is well defined a.e. in Hn and |DsHu|p∈L1(Hn) thanks to Tonelli's theorem.
From now on we fix 0<s<1, 1<p<∞ with sp<Q. Then, the following result holds true.
Theorem 1.3. Let (uk)k be a sequence in HWs,p(Hn) such that uk⇀u in HWs,p(Hn), and furthermore|uk|p∗sdξ∗⇀ν, |DsHuk|pdξ∗⇀μ, in M(Hn), for some appropriateu∈HW1,p(Hn), and finite nonnegative Radon measures μ, ν on Hn.
Then, there exist an at most countable set J, a family of points {ξj}j∈J⊂Hn, two families of nonnegative numbers {μj}j∈J and {νj}j∈Jsuch that
ν=|u|p∗sdξ+∑j∈Jνjδξj,μ≥|DsHu|pdξ+∑j∈Jμjδξj, | (1.18) |
νp/p∗sj≤μjCp∗sforallj∈J, | (1.19) |
where the constant Cp∗s is defined in (1.16).
In the Euclidean setting, the first extension of the CC method in the fractional Sobolev spaces was obtained in [30] for p=2 and then in [28] for any p, with 1<p<N/s. We also refer to [4,10,12] for similar results in this context and to [33] for the vectorial fractional Sobolev spaces.
In Theorem 1.3 we extend the previous results from the Euclidean setting to the Heisenberg environment and we also widen Theorem A from the local case to the fractional setup. To the best of our knowledge Theorem 1.3 is the first extension of the method in the fractional Sobolev space in Heisenberg group.
Actually, the strategy is the same as the one in the seminal papers of Lions [25,26], but there are several complications due to both the nonlocal and the subelliptic context. In order to overcome these difficulties, we employ the crucial Lemmas 4.4 and 4.5, proved using the key Lemma 4.2 and Corollary 4.3. To enter into details, the latter results give precise decay estimates and scaling properties for the fractional (s,p) horizontal gradients of functions of class C∞c(Hn), with respect to the intrinsic family of dilations δR.
The paper is organized as follows. In Section 2, we recall some fundamental definitions and properties related to the Heisenberg group Hn. Section 3 is devoted to the proof of Theorems 1.1 and 1.2, while the final Section 4 deals with the proof of Theorem 1.3, based on some preliminary lemmas.
In this section we present the basic properties of Hn as a Lie group. For a complete treatment, we refer to [13,16,17,20,35]. Let Hn be the Heisenberg group of topological dimension 2n+1, that is the Lie group which has R2n+1 as a background manifold and whose group structure is given by the non–Abelian law
ξ∘ξ′=(z+z′,t+t′+2n∑i=1(yix′i−xiy′i)) |
for all ξ, ξ′∈Hn, with
ξ=(z,t)=(x1,…,xn,y1,…,yn,t)andξ′=(z′,t′)=(x′1,…,x′n,y′1,…,y′n,t′). |
The inverse is given by ξ−1=−ξ and so (ξ∘ξ′)−1=(ξ′)−1∘ξ−1.
The real Lie algebra of Hn is generated by the left–invariant vector fields on Hn
Xj=∂∂xj+2yj∂∂t,Yj=∂∂yj−2xj∂∂t,T=∂∂t, |
for j=1,…,n. This basis satisfies the Heisenberg canonical commutation relations
[Xj,Yk]=−4δjkT,[Yj,Yk]=[Xj,Xk]=[Yj,T]=[Xj,T]=0. |
Moreover, all the commutators of length greater than two vanish, and so Hn is a nilpotent graded stratified group of step two. A left invariant vector field X, which is in the span of {Xj,Yj}nj=1, is called horizontal.
For each real positive number R, the dilation δR:Hn→Hn, naturally associated with the Heisenberg group structure, is defined by
δR(ξ)=(Rz,R2t)for all ξ=(z,t)∈Hn. |
It is easy to verify that the Jacobian determinant of the dilatation δR is constant and equal to R2n+2, where the natural number Q=2n+2 is the homogeneous dimension of Hn.
The anisotropic dilation structure on Hn introduces the Korányi norm, which is given by
r(ξ)=r(z,t)=(|z|4+t2)1/4for all ξ=(z,t)∈Hn. |
Consequently, the Korányi norm is homogeneous of degree 1, with respect to the dilations δR, R>0, that is
r(δR(ξ))=r(Rz,R2t)=(|Rz|4+R4t2)1/4=Rr(ξ)for all ξ=(z,t)∈Hn. |
Clearly, δR(η∘ξ)=δR(η)∘δR(ξ). The corresponding distance, the so called Korányi distance, is
dK(ξ,ξ′)=r(ξ−1∘ξ′)for all (ξ,ξ′)∈Hn×Hn. |
Let BR(ξ0)={ξ∈Hn:dK(ξ,ξ0)<R} be the Korányi open ball of radius R centered at ξ0. For simplicity we put BR=BR(O), where O=(0,0) is the natural origin of Hn.
The Lebesgue measure on R2n+1 is invariant under the left translations of the Heisenberg group. Thus, since the Haar measures on Lie groups are unique up to constant multipliers, we denote by dξ the Haar measure on Hn that coincides with the (2n+1)–Lebesgue measure and by |U| the (2n+1)–dimensional Lebesgue measure of any measurable set U⊆Hn. Furthermore, the Haar measure on Hn is Q–homogeneous with respect to dilations δR. Consequently,
|δR(U)|=RQ|U|,d(δRξ)=RQdξ. |
In particular, |BR(ξ0)|=|B1|RQ for all ξ0∈Hn.
We define the horizontal gradient of a C1 function u:Hn→R by
DHu=n∑j=1[(Xju)Xj+(Yju)Yj]. |
Clearly, DHu∈span{Xj,Yj}nj=1. In span{Xj,Yj}nj=1≃R2n we consider the natural inner product given by
(X,Y)H=n∑j=1(xjyj+˜xj˜yj) |
for X={xjXj+˜xjYj}nj=1 and Y={yjXj+˜yjYj}nj=1. The inner product (⋅,⋅)H produces the Hilbertian norm
|X|H=√(X,X)H |
for the horizontal vector field X.
For any horizontal vector field function X=X(ξ), X={xjXj+˜xjYj}nj=1, of class C1(Hn,R2n), we define the horizontal divergence of X by
divHX=n∑j=1[Xj(xj)+Yj(˜xj)]. |
Similarly, if u∈C2(Hn), then the Kohn–Spencer Laplacian in Hn, or equivalently the horizontal Laplacian, or the sub–Laplacian, of u is
ΔHu=n∑j=1(X2j+Y2j)u=n∑j=1(∂2∂x2j+∂2∂y2j+4yj∂2∂xj∂t−4xj∂2∂yj∂t)u+4|z|2∂2u∂t2. |
According to the celebrated Theorem 1.1 due to Hörmander in [19], the operator ΔH is hypoelliptic. In particular, ΔHu=divHDHu for each u∈C2(Hn). A well known generalization of the Kohn–Spencer Laplacian is the horizontal p–Laplacian on the Heisenberg group, p∈(1,∞), defined by
ΔH,pφ=divH(|DHφ|p−2HDHφ) for all φ∈C∞c(Hn). |
Let us now review some useful facts about the classical Sobolev spaces on the Heisenberg group Hn. We just consider the special case in which 1≤p<Q and Ω is an open set in Hn. Denote by HW1,p(Ω) the horizontal Sobolev space consisting of the functions u∈Lp(Ω) such that DHu exists in the sense of distributions and |DHu|H∈Lp(Ω), endowed with the natural norm
‖u‖HW1,p(Ω)=(‖u‖pLp(Ω)+‖DHu‖pLp(Ω))1/p,‖DHu‖Lp(Ω)=(∫Ω|DHu|pHdξ)1/p. |
By [14] we know that if 1≤p<Q, then the embedding
HW1,p(Ω)↪Ls(Ω) for all s∈[p,p∗],p∗=pQQ−p, |
is continuous.
Let us also briefly recall a version of the Rellich theorem in the Heisenberg group. This topic is largely treated in [13,16,17,20] for vector fields satisfying the Hörmander condition. The general Hörmander vector fields have been introduced in [19] and include, as a special case, the horizontal vector fields (1.2) on the Heisenberg group. For our purposes it is sufficient to recall that for any p, with 1≤p<Q, and for any Korányi ball BR(ξ0), the embedding
HW1,p(BR(ξ0))↪↪Lq(BR(ξ0)) | (2.1) |
is compact, provided that 1≤q<p∗. This result holds, more in general, for bounded Poincaré–Sobolev domains Ω of Hn and was first established in [27], even for general Hörmander vector fields. For a complete treatment on this topic we mention, e.g., [16,20,25].
Let s∈(0,1) and 1<p<∞. We endow HWs,p(Hn), defined in the Introduction, with the norm
‖⋅‖HWs,p(Hn)=‖⋅‖p+[⋅]H,s,p. |
Our aim is to prove the compactness of the immersion HWs,p(Hn)↪Lp(BR(ξ0)) for all ξ0∈Hn and R>0. The proof relies on a Lie group version of the celebrated Frèchet–Kolmogorov Compactness Theorem, cf. Theorem A.4.1 of [11]. First, we need the following lemma.
Lemma 2.1. Let 0<s<1, 1<p<∞.Then, there exists a constant C=C(s,p,n)>0 such that for any h∈Hn, with 0<r(h)<1/2,
‖τhu−u‖p≤Cr(h)s[u]H,s,pforallu∈HWs,p(Hn), |
where τhu(ξ)=u(h∘ξ) for ξ∈Hn.
Proof. Fix u∈HWs,p(Hn), h∈Hn, with 0<r(h)<1/2, and ξ∈Hn. Take any η∈B(ξ,r(h)). Let us first observe that r(η−1∘ξ)≤r(h), so that r(η−1∘h∘ξ)≤r(η−1∘ξ)+r(h)≤2r(h) by the triangle inequality. Then,
|τhu(ξ)−u(ξ)|p≤2p−1(|u(h∘ξ)−u(η)|p+|u(η)−u(ξ)|p). |
Now, averaging in η over B(ξ,r(h)), we get
|τhu(ξ)−u(ξ)|p≤c(1r(h)Q∫B(ξ,r(h))|u(h∘ξ)−u(η)|pdη+1r(h)Q∫B(ξ,r(h))|u(η)−u(ξ)|pdη), |
with c=c(s,p,n). Thus, integrating in ξ over Hn, we obtain
‖τhu−u‖p≤cr(h)sp(∫Hn∫B(ξ,r(h))|u(h∘ξ)−u(η)|pr(h)Q+spdηdξ+∫Hn∫B(ξ,r(h))|u(η)−u(ξ)|pr(h)Q+spdηdξ)≤2cr(h)sp(∫Hn∫B(ξ,r(h))|u(h∘ξ)−u(η)|pr(η−1∘h∘ξ)Q+spdηdξ+∫Hn∫B(ξ,r(h))|u(η)−u(ξ)|pr(η−1∘ξ)Q+spdηdξ)≤Cr(h)sp[u]H,s,p, |
with C=4c.
Theorem 2.2. Let 0<s<1 and 1<p<∞. Then, for every sequence (uk)k bounded in HWs,p(Hn) there exists u∈HWs,p(Hn) and a subsequence (ukj)j⊂(uk)k such thatfor all ξ0∈Hn and R>0
ukj→uinLp(BR(ξ0))asj→∞. |
Proof. Let M=supk∈N‖uk‖HWs,p(Hn). Clearly, if (uk)k is bounded in HWs,p(Hn), then is also bounded in Lp(Hn). Moreover, by Lemma 2.1, we know that
‖τhuk−uk‖p≤Cr(h)s[uk]H,s,p≤CMr(h)s. |
Consequently,
limh→Osupk∈N‖τhuk−uk‖p=0. |
Therefore, a Lie group version of the Fréchet–Kolmogorov theorem, cf. Theorem A.4.1 of [11], yields the existence of a function u∈Lp(Hn) and a subsequence of (uk)k, still denoted (uk)k, such that uk→u a.e. in Hn and uk→u in Lp(BR(ξ0)) for all ξ0∈Hn and R>0.
It remains to prove that u∈HWs,p(Hn). This follows straightly from an application of Fatou's Lemma. Indeed,
0≤limk→∞|uk(η)−uk(ξ)|pr(η−1∘ξ)Q+sp=|u(η)−u(ξ)|pr(η−1∘ξ)Q+sp for a.e. (ξ,η)∈Hn×Hn. |
Consequently, Fatou's Lemma, together with the lower semicontinuity of [⋅]H,s,p, gives
[u]H,s,p≤limk→∞[uk]H,s,p≤supk∈N[uk]H,s,p<∞. |
This concludes the proof.
This section is devoted to the proof of Theorems 1.1 and 1.2.
Proof of Theorem 1.1. Let (uk)k be a sequence in S1,p(Hn) as in the statement of the theorem. Obviously, (1.7), (1.8) and the first part of (1.10) follow from Theorem A, see [20]. Thus, there is no reason to repeat the proof here. Let us then focus on the proof of (1.9). We proceed diving the argument into two cases.
Case 1. u=0. Fix φ∈C∞c(Hn). Then, since clearly φuk∈S1,p(Hn) for all k, we get by (1.6)
Hp‖φuk‖pHp≤∫Hn|φ|p|DHuk|pHdξ+‖DHφuk‖pp. | (3.1) |
Now, by the subelliptic Rellich Theorem, see (2.1), we know that uk→0 in Lp(BR) for all R>0. Therefore,
limk→∞‖DHφuk‖p=0. | (3.2) |
Consequently, by the weak∗ convergence and (3.2), letting k→∞, we obtain
(∫Hn|φ|pdω)1/p≤H−1/pp(∫Hn|φ|pdμ)1/pfor all φ∈C∞c(Hn). |
Thus, by Lemma 1.4.6 of [20], we conclude that there exist an at most countable set J, a family of points {ξj}j∈J⊂Hn and a family of nonnegative numbers {ωj}j∈J∪{0}, such that
ω=ω0δO+∑j∈Jωjδξj. | (3.3) |
Clearly, the set J determined in (3.3) is not necessary the same of the one obtained in the representation of ν. However, since the coefficients νj,μj,ωj are allowed to be 0, we can replace these two sets with their union (which is still at most countable). For this reason we keep the same notation J for the index set.
In order to conclude the proof of (1.9) on Case 1, it remains to show that ω is concentrated at O, namely that ωj=0 for any j∈J. But this is obvious. Indeed, fix φ∈C∞c(Hn), with O∉suppφ, so that ξ↦|φ(ξ)|pψpr(ξ)−p is in L∞(suppφ). Then, since obviously uk→0 in Lp(suppφ), we get
∫Hn|φ|p|uk|pψpdξr(ξ)p=∫suppφ|φ|p|uk|pψpdξr(ξ)p≤C∫suppφ|uk|pdξ→0 |
as k→∞. This, combined with the weak∗ convergence, gives ∫Hn|φ|pdω=0, that is ω is a measure concentrated in O. Hence ω=ω0δO, and so (1.9) in proved in Case 1.
Case 2. u≠0. Set ˜uk=uk−u. Clearly, ˜uk⇀0 in S1,p(Hn) and (3.1) still holds for φ˜uk for any φ∈C∞c(Hn). Moreover, thanks to Case 1, there exists a finite nonnegative Radon measure ˜ω on Hn, such that, up to a subsequence still labelled (˜uk)k, we have as k→∞
|˜uk|pψpdξr(ξ)p∗⇀˜ωin M(Hn), | (3.4) |
where
˜ω=ω0δO, | (3.5) |
and ω0 is an appropriate nonnegative number as shown in Case 1. Now, by (2.1), up to a subsequence,
uk→ua.e. in Hn,|uk|≤gRa.e. in Hn |
for some gR∈Lp(BR) and all R>0. Thus, for all φ∈C∞c(Hn) an application of Brézis–Lieb lemma yields
limk→∞(‖φuk‖pHp−‖φ~uk‖pHp)=‖φu‖pHp. |
A combination of the above formulas gives for all φ∈C∞c(Hn)
|uk|pψpdξr(ξ)p−|u|pψpdξr(ξ)p=|uk−u|pψpdξr(ξ)p−o(1), | (3.6) |
where o(1)∗⇀0 in M(Hn). Then, computing the limit in (3.6), by the weak∗ convergence and (3.4), we get ˜ω=ω−|uk−u|pψpdξr(ξ)p. Consequently, taking into account (3.5), we obtain (1.9).
It remains to prove that νp/p∗0≤(μ0−σω0)/Iσ. Fix φ∈C∞c(Hn) such that 0≤φ≤1, φ(O)=1 and supp φ=¯B1. Take ε>0 and put φε(ξ)=φ(δ1/ε(ξ)), ξ∈Hn. Then,
Iσ(∫Hn|φε|p∗|uk|p∗dξ)p/p∗≤∫Hn|φε|p|DHuk|pHdξ+‖DHφεuk‖pp−σ∫Hn|φε|p|uk|pψpdξr(ξ)p=∫Hn|φε|p|DHuk|pHdξ+o(1)−σ∫Hn|φε|p|uk|pψpdξr(ξ)p, | (3.7) |
arguing as before. Now, we know that
limε→0+limk→∞∫Hn|φε|p∗|uk|p∗dξ=ν0, | (3.8) |
and
limε→0+limk→∞∫Hn|φε|p|DHuk|pHdξ=μ0. | (3.9) |
Finally, from (1.9) and the fact that ω≥ω0δO we get
limε→0+limk→∞∫Hn|φε|p|uk|pψpdξr(ξ)p=limε→0+∫Bε|φε|pdω≥ω0. | (3.10) |
Hence, passing to the limit as k→∞ and ε→0+ in (3.7), by (3.8)–(3.10) we obtain that
Iσνp/p∗0≤μ0−σω0. |
This concludes the proof.
In Theorem 1.1 we examine the behavior of weakly convergent sequences in the Folland–Stein space in situations in which the lack of compactness occurs. However, this method does not exclude a possible loss of compactness due to the drifting towards infinity of the mass, or – in other words – the concentration at infinity. Let us then turn to the proof of the concentration–compactness principle at infinity, which extend the method introduced in the Euclidean setting in [3,6].
Proof of Theorem 1.2. Fix a sequence (uk)k in S1,p(Hn), as in the statement of the Theorem 1.1.
Let Ψ∈C∞(Hn) be such that 0≤Ψ≤1, Ψ=0 in B1 and Ψ=1 in Bc2. Take R>0 and put ΨR(ξ)=Ψ(δ1/R(ξ)), ξ∈Hn. Write
∫Hn|DHuk|pHdξ=∫Hn|DHuk|pH|ΨR|pdξ+∫Hn|DHuk|pH(1−|ΨR|p)dξ. | (3.11) |
We first observe that
∫Bc2R|DHuk|pHdξ≤∫Hn|DHuk|pH|ΨR|pdξ≤∫BcR|DHuk|pHdξ |
and so by (1.11)
μ∞=limR→∞lim supk→∞∫Hn|DHuk|pH|ΨR|pdξ. | (3.12) |
On the other hand, since μ is finite, 1−|ΨR|p has compact support and ΨR→0 a.e. in Hn, we have by the definition of μ and the dominated convergence theorem that
limR→∞lim supk→∞∫Hn|DHuk|pH(1−|ΨR|p)dξ=limR→∞∫Hn(1−|ΨR|p)dμ=μ(Hn). | (3.13) |
Using (3.12) and (3.13) in (3.11) we obtain the second part of (1.13). Arguing similarly for ν and ω, we see that
ν∞=limR→∞lim supk→∞∫Hn|ΨR|p∗|uk|p∗dξ,ω∞=limR→∞lim supk→∞∫Hn|ΨR|p|uk|pψpdξr(ξ)p, | (3.14) |
and
limR→∞lim supk→∞∫Hn(1−|ΨR|p∗)|uk|p∗dξ=ν(Hn),limR→∞lim supk→∞∫Hn(1−|ΨR|p)|uk|pψpdξr(ξ)p=ω(Hn). |
Thus, (1.13)–(1.14) are proved in the same way.
In order to show the last part of (1.14), let us consider again the regular function ΨR. Then, since 0≤ΨR≤1, by (1.5) applied to ΨRuk∈S1,p(Hn), we get for all k
Iσ(∫Hn|ΨR|p∗|uk|p∗dξ)p/p∗≤∫Hn|ΨR|p|DHuk|pHdξ+‖DHΨRuk‖pp−σ∫Hn|ΨR|p|uk|pψpdξr(ξ)p. | (3.15) |
Finally, from the fact that limR→∞lim supk→∞‖DHΨRuk‖pp=0, using (3.12) and (3.14) in (3.15) we obtain the desired conclusion.
This section is devoted to the proof of Theorem 1.3. Before getting there, we need some preliminary results.
Lemma 4.1. Let φ∈C∞c(Hn), ε>0 and ξ0∈Hn. Define Hn∋ξ↦φε(ξ)=φ(δ1/ε(ξ−10∘ξ)). Then,
|DsHφε(ξ)|p=1εsp|DsHφ(δ1/ε(ξ−10∘ξ))|p. |
Proof. Fix φ∈C∞c(Hn), ε>0 and ξ0∈Hn. The proof is a simple consequence of the change of variables formula. Indeed, if we put η=δ1/ε(h), dη=ε−Qdh, then
|DsHφε(ξ)|p=∫Hn|φε(ξ∘h)−φε(ξ)|pr(h)Q+psdh=∫Hn|φ(δ1/ε(ξ−10∘ξ∘h))−φ(δ1/ε(ξ−10∘ξ))|pr(h)Q+psdh=∫Hn|φ(δ1/ε(ξ−10∘ξ)∘δ1/ε(h))−φ(δ1/ε(ξ−10∘ξ))|pr(h)Q+psdh=1εsp∫Hn|φ(δ1/ε(ξ−10∘ξ∘η)−φ(δ1/ε(ξ−10∘ξ))|pr(η)Q+psdη, |
as required thanks to (1.17).
Note that, in general, the nonlocal (s,p) horizontal gradient of a compactly supported function does not need to have compact support. For this, we use the following lemma, which gives valuable decay estimates of the fractional (s,p) horizontal gradient of a C∞c(Hn) function as r(ξ)→∞. The next lemma is an extension of Lemma 2.2 of [4] to the Heisenberg setting.
Lemma 4.2. Let φ∈C∞c(Hn) be such that 0≤φ≤1, suppφ⊂BR for some R>0.Then, there exists a constant C=C(s,p,n) such that for any ξ∈Hn
|DsHφ(ξ)|p≤Cmin{1,RQr(ξ)−(Q+sp)}. |
In particular, |DsHφ|p∈L∞(Hn).
Proof. Let us first prove the global L∞ bound. Consider for any ξ∈Hn
|Dsφ(ξ)|p=∫Hn|φ(ξ∘h)−φ(ξ)|pr(h)Q+psdh=(∫B1+∫Bc1)|φ(ξ∘h)−φ(ξ)|pr(h)Q+psdh |
and compute separately the last two integrals. By the mean value theorem and [14]
∫B1|φ(ξ∘h)−φ(ξ)|pr(h)Q+psdh≤C1∫B11r(h)Q+sp−pdh≤C2, |
since Q+sp−p<Q. On the other hand,
∫Bc1|φ(ξ∘h)−φ(ξ)|pr(h)Q+psdh≤2p‖φ‖p∞∫Bc11r(h)Q+spdh≤C3, |
being obviously Q+sp>Q. Now, consider ξ∈Hn with r(ξ)≥2R. Clearly, φ(ξ)=0 and so
|Dsφ(ξ)|p=∫Hn|φ(ξ∘h)|pr(h)Q+psdh=∫r(ξ∘h)<R|φ(ξ∘h)|pr(h)Q+psdh. |
Now, if r(ξ∘h)<R and r(ξ)>2R, then r(ξ)−r(h)≤r(ξ∘h) so that r(h)≥r(ξ)−R≥r(ξ)/2. Therefore,
|Dsφ(ξ)|p≤2Q+spr(ξ)Q+sp‖φ‖p∞∫r(ξ∘h)<Rdh≤CRQr(ξ)−(Q+sp). |
This concludes the proof of the lemma.
Combining Lemma 4.1 and Lemma 4.2, we obtain the following.
Corollary 4.3. Let φ∈C∞c(Hn) be such that 0≤φ≤1 and suppφ⊂B1. Let ξ0∈Hn and define Hn∋ξ↦φε(ξ)=φ(δ1/ε(ξ−10∘ξ)). Then, there exists a constant C=C(s,p,n) such that for any ξ∈Hn
|DsHφε(ξ)|p≤Cmin{ε−sp,εQr(ξ)−(Q+sp)}. |
Using the previous estimates, we are able to prove the next result, which is an extension of Lemma 2.4 of [4] to the Heisenberg context.
Lemma 4.4. Let φ∈C∞c(Hn).Then, the following embedding is compact
HWs,p(Hn)↪↪Lp(Hn,|DsHφ|pdξ). |
Proof. Let (uk)k be a bounded sequence in HWs,p(Hn), say supk‖uk‖HWs,p(Hn)≤M, with M>0. From the reflexivity of HWs,p(Hn) and Theorem 2.2, there exist u∈HWs,p(Hn) and a subsequence, still denoted by (uk)k, such that
uk⇀u in HWs,p(Hn),uk→u∈Lp(BR)for any R>0. | (4.1) |
Fix R>0 so large that suppφ⊂BR. Certainly,
∫Hn|uk(ξ)−u(ξ)|p|DsHφ(ξ)|pdξ=(∫B2R+∫Bc2R)|uk(ξ)−u(ξ)|p|DsHφ(ξ)|pdξ. |
Now, from Lemma 4.2 and (4.1)
∫B2R|uk(ξ)−u(ξ)|p|DsHφ(ξ)|pdξ≤‖DsHφ‖p∞∫B2R|uk(ξ)−u(ξ)|pdξ=o(1) | (4.2) |
as k→∞. On the other hand, using (1.16) and the Hölder inequality with q=p∗s/p=Q/(Q−sp) and q′=Q/sp, we get by Lemma 4.2
∫Bc2R|uk(ξ)−u(ξ)|p|DsHφ(ξ)|pdξ≤‖uk−u‖pp∗s(∫Bc2R|∫Hn|φ(ξ∘h)−φ(ξ)|pr(h)Q+psdh|Q/sdξ)sp/Q≤2p2−1‖φ‖p2∞(‖uk‖pp∗s+‖u‖pp∗s)(∫Bc2Rdξr(ξ)Q(1+Q/sp))sp/Q≤C(∫Bc2Rdξr(ξ)Q(1+Q/sp))sp/Q | (4.3) |
where C=2p2‖φ‖p2∞Mp. Now, for any τ>0 we can choose R>0 ever larger, if necessary, so that
(∫Bc2Rdξr(ξ)Q(1+Q/sp))sp/Q<τC. |
Finally, by (4.2) and (4.3), we obtain
lim supk→∞∫Hn|uk(ξ)−u(ξ)|p|DsHφ(ξ)|pdξ≤τ |
for all τ>0. Sending τ→0+ we get the desired conclusion.
The proof of the next lemma is based on the precise decay rate of |DsHφε|p, cf. Corollary 4.3. The main difficulty here, as we already pointed out in the Introduction, is based essentially on the fact that the nonlocal (s,p) horizontal gradient |DsHφε|p does not need to have compact support. The proof uses the same strategy of Lemma 4.4, which is effective thanks to the decay estimates given in Lemma 4.1 and Corollary 4.3.
Lemma 4.5. Let (uk)k be a bounded sequence in HWs,p(Hn) and let φ∈C∞c(Hn) besuch that 0≤φ≤1, φ(O)=1 andsupp φ⊂B1. Take ε>0, fix ξ0∈Hn and putHn∋ξ↦φε(ξ)=φ(δ1/ε(ξ−10∘ξ)). Then
limε→0+lim supk→∞∫Hn|DsHφε|p|uk|pdξ=0. |
Proof. Let (uk)k be a bounded sequence in HWs,p(Hn), say supk‖uk‖HWs,p(Hn)=M. From the reflexivity of HWs,p(Hn) and Theorem 2.2, there exist u∈HWs,p(Hn) and a subsequence, still denoted by (uk)k, such that
uk⇀u in HWs,p(Hn),uk→u in Lp(BR(ξ0)), | (4.4) |
for any R>0 and ξ0∈Hn. Clearly,
∫Hn|DsHφε|p|uk|pdξ=(∫Bε(ξ0)+∫Bcε(ξ0))|DsHφε|p|uk|pdξ. |
Let us first estimate the integral over Bε(ξ0). By Corollary 4.3 there exists C=C(s,p,n)>0 such that
lim supk→∞∫Bε(ξ0)|DsHφε|p|uk|pdξ≤Clim supk→∞ε−sp∫Bε(ξ0)|uk|p=Cε−sp∫Bε(ξ0)|u|pdξ⟶ε→0+0 | (4.5) |
thanks to (4.4) and the Lebesgue theorem, being sp<Q.
Now we turn to the integral over Bcε(ξ0). Using the Hölder inequality with q=p∗s/p=Q/(Q−sp) and q′=Q/sp, and again Corollary 4.3, we get
∫Bcε(ξ0)|DsHφε|p|uk|pdξ≤‖uk‖pp∗s(∫Bcε(ξ0)|DsHφε(ξ)|Q/sdξ)sp/Q≤CMpεQ(∫Bcεdξr(ξ)Q(1+Q/sp))sp/Q≤CMp|B1|sp/QεQ(εQ(1−Q/sp))sp/Q=CMp|B1|sp/Qεsp. |
Therefore, it follows that
lim supk→∞∫Bcε|DsHφε|p|uk|pdξ≤CMp|B1|sp/Qεsp. | (4.6) |
Finally, using (4.5) and (4.6), we conclude
limε→0+lim supk→∞∫Hn|DsHφε,j|p|uk|pdξ≤limε→0+(Cε−sp∫Bε(ξ0)|u|pdξ+CMp|B1|sp/Qεsp)=0, |
as required.
Lemma 4.5 extends to the Heisenberg case a remark given in the proof of Theorem 1.1 of [4], stated in the Euclidean framework.
Proof of Theorem 1.3. Let (uk)k be a sequence in HWs,p(Hn), as in the statement of the theorem, and let us divide the proof into two cases.
Case 1. u=0. Fix φ∈C∞c(Hn). Then, an application of Lemma 4.4 immediately yields
∫∫Hn×Hn|uk(ξ)|p|φ(ξ)−φ(η)|pr(η−1∘ξ)Q+spdξdη=o(1), |
as k→∞. Consequently, since φuk∈HWs,p(Hn) for all k, we get
Cp∗s(∫Hn|φ|p∗s|uk|p∗sdξ)p/p∗s≤[φuk]pH,s,p=(∫∫Hn×Hn|(φuk)(ξ)−(φuk)(η)|pr(η−1∘ξ)Q+spdξdη)≤2p−1(∫∫Hn×Hn|φ(η)|p|uk(ξ)−uk(η)|pr(η−1∘ξ)Q+spdξdη+∫∫Hn×Hn|uk(ξ)|p|φ(ξ)−φ(η)|pr(η−1∘ξ)Q+spdξdη)≤2p−1∫Hn|φ|p|DsHuk|pdξ+o(1) | (4.7) |
as k→∞. Therefore, passing to the limit in (4.7), by the weak∗ convergence we have the following reverse Hölder inequality
(∫Hn|φ|p∗sdν)1/p∗s≤C(∫Hn|φ|pdμ)1/qfor all φ∈C∞c(Hn). |
Thus, by Lemma 1.4.6 of [20], we conclude that there exist an at most countable set J, a family of points {ξj}j∈J⊂Hn and a family of nonnegative numbers {νj}j∈J such that
ν=∑j∈Jνjδξj | (4.8) |
Case 2. u≠0. Set ˜uk=uk−u. Clearly, ˜uk⇀0 in HWs,p(Hn) and (4.7) still holds for φ˜uk for any φ∈C∞c(Hn). Moreover, k↦|˜uk|p∗sdξ and k↦|DsH˜uk|pdξ are still bounded sequences of measures and so by Proposition 1.202 of [15], we can conclude that there exist two bounded nonnegative Radon measure ˜ν and ˜μ on Hn, such that, up to a subsequence, we have
|DsH˜uk|pdξ∗⇀˜μ,|˜uk|p∗sdξ∗⇀˜ν in M(Hn). | (4.9) |
Thus, from Case 1 there exist an at most countable set J, a family of points {ξj}j∈J⊂Hn and a family of nonnegative numbers {νj}j∈J such that ˜ν=∑j∈Jνjδξj. Consequently, the claimed representation (1.7) of ν follows exactly as in Theorem 1.1.
Let us now prove the first part of (1.10). Fix a test function φ∈C∞c(Hn), such that 0≤φ≤1, φ(O)=1 and supp φ⊂B1. Take ε>0 and put φε,j(ξ)=φ(δ1/ε(ξ−1j∘ξ)), ξ∈Hn, for any fixed j∈J, where {ξj}j is introduced in (1.7). Fix j∈J and τ>0. Then, there exists Cτ>0 such that, by (1.16) applied to φε,juk, we have
Cp∗s(∫Hn|φε,j|p∗s|uk|p∗sdξ)p/p∗s≤∫∫Hn×Hn|(φε,juk)(ξ)−(φε,juk)(η)|pr(η−1∘ξ)Q+spdξdη≤(1+τ)∫Hn|φε,j|p|DsHuk|pdξ+Cτ∫Hn|DsHφε,j|p|uk|pdξ. | (4.10) |
We aim to pass to the limit in (4.10) as k→∞ and ε→0+. To do this, let us observe first that from the weak∗ convergence and (1.7) we get
limε→0+limk→∞∫Hn|φε,j|p∗s|uk|p∗sdξ=limε→0+∫Bε(ξj)|φε,j|p∗sdν=limε→0+{∫Bε(ξj)|φε,j|p∗s|u|p∗sdξ+νjδξj(φε,j)}=νj, | (4.11) |
since
∫Bε(ξj)|φε,j|p∗s|u|p∗sdξ≤∫Bε(ξj)|u|p∗sdξ=o(1) |
as ε→0+, being 0≤φ≤1. On the other hand, the weak∗ convergence gives
limk→∞∫Hn|φε,j|p|DsHuk|pdη=∫Hn|φε,j|pdμ, | (4.12) |
while Lemma 4.5 yields
limε→0+lim supk→∞∫Hn|DsHφε,j|p|uk|pdη=0. | (4.13) |
Then, combining (4.11)–(4.13) and letting ε→0+ in (4.10), we find that
Cp∗sνp/p∗sj≤(1+τ)μj for any j∈J, |
where μj=limε→0+μ(Bε(ξj)). Since τ>0 is arbitrary, sending τ→0+, we finally obtain
Cp∗sνp/p∗sj≤μj,j∈J. |
Obviously,
μ≥∑j∈Jμjδξj. |
Denote by Bε(ξ0) the Euclidean ball of R2n+1 of center ξ0∈Hn and radius ε. By Lebesgue's differentiation theorem for measures (see for example [15]), in order to prove that μ≥|DsHu|pdξ it suffices to show that
lim infε→0+μ(Bε(ξ0)|Bε(ξ0)|≥|DsHu|p(ξ0)for a.e.ξ0∈Hn, | (4.14) |
where |Bε(ξ0)| is the Lebesgue measure of the Euclidean ball Bε(ξ0).
Clearly, since |DsHu|pdξ∈L1loc(Hn), we know that for a.e. ξ0∈Hn
limε→0+1|Bε(ξ0)|∫Bε(ξ0)|DsHu|p(ξ)dξ=|DHu|pH(ξ0). | (4.15) |
Fix ε>0 and ξ0∈Hn such that (4.15) holds. Now, the functional Φ:HWs,p(Hn)→R, defined as
Φu=∫Bε(ξ0)∫Hn|u(ξ)−u(η)|pr(η−1∘ξ)Q+spdηdξ=∫Bε(ξ0)|DsHu|pdξ, |
is convex and strongly continuous on HWs,p(Hn). Thus, since uk⇀u in HWs,p(Hn), we have
lim infk→∞∫Bε(ξ0)|DsHuk|pdξ≥∫Bε(ξ0)|DsHu|pdξ. |
Therefore, an application of Proposition 1.203 – Part (ii) of [15] gives
μ(Bε(ξ0)|Bε(ξ0)|≥lim supk→∞μk(Bε(ξ0))|Bε(ξ0)|=lim supk→∞1|Bε(ξ0)|∫Bε(ξ0)|DsHuk|pdξ≥lim infk→∞1|Bε(ξ0)|∫Bε(ξ0)|DsHuk|pdξ≥1|Bε(ξ0)|∫Bε(ξ0)|DsHu|pdξ. |
Now, passing to the liminf as ε→0+ and using (4.15), we obtain (4.14).
Finally, since |DsHu|pdξ is orthogonal to ∑j∈Jμjδξj, we get the desired conclusion. This concludes the proof.
The authors are members of the Gruppo Nazionale per l'Analisi Matematica, la Probabilità e le loro Applicazioni (GNAMPA) of the Istituto Nazionale di Alta Matematica (INdAM). P. Pucci was partly supported by the INdAM – GNAMPA Project Equazioni alle derivate parziali: problemi e modelli (Prot_U-UFMBAZ-2020-000761) and also by the Fondo Ricerca di Base di Ateneo – Esercizio 2017–2019 of the University of Perugia, named PDEs and Nonlinear Analysis.
All authors declare no conflicts of interest in this paper.
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