Citation: Yoram Baram. Asynchronous Segregation of Cortical Circuits and Their Function: A Life-long Role for Synaptic Death[J]. AIMS Neuroscience, 2017, 4(2): 87-101. doi: 10.3934/Neuroscience.2017.2.87
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The Ricci curvature in Finsler geometry naturally generalizes the Ricci curvature in Riemannian geometry. However, in Finsler geometry, there are several versions of the definition of scalar curvature because the Ricci curvature tensor is defined in different forms. Here we adopt the definition of scalar curvature, which was introduced by Akbar–Zadeh [1,see (2.1)]. Tayebi [11] characterized general fourth-root metrics with isotropic scalar curvature. Moreover, he studied Bryant metrics with isotropic scalar curvature. Later, a locally conformally flat (α,β)-metric with weakly isotropic scalar curvature was studied by Chen–Xia [4]. They proved that its scalar curvature must vanish. Recently, Cheng–Gong [5] proved that if a Randers metric is of weakly isotropic scalar curvature, then it must be of isotropic S-curvature. Furthermore, they concluded that when a locally conformally flat Randers metric is of weakly isotropic scalar curvature, it is Minkowskian or Riemannian. Very recently, Ma–Zhang–Zhang [8] showed that the Kropina metric with isotropic scalar curvature is equivalent to an Einstein Kropina metric according to the navigation data.
Shimada [9] first developed the theory of m-th root metrics as an interesting example of Finsler metrics, immediately following Matsumoto and Numata's theory of cubic metrics [7]. It is applied to biology as an ecological metric by Antonelli [2]. Later, many scholars studied these metrics ([3,6,10,11,12], etc). In [13], cubic Finsler manifolds in dimensions two or three were studied by Wegener. He only abstracted his PhD thesis and barely did all the calculations in that paper. Kim and Park [6] studied the m-th root Finsler metrics which admit (α,β)−types. In [12], Tayebi–Razgordani–Najafi showed that if the locally conformally flat cubic metric is of relatively isotropic mean Landsberg curvature on a manifold M of dimension n(≥3), then it is a Riemannian metric or a locally Minkowski metric. Tripathia–Khanb–Chaubey [10] considered a cubic (α,β)-metric which is a special class of p-power Finsler metric, and obtained the conditions under which the Finsler space with such special metric will be projectively flat. Further, they obtained in which case this Finsler space will be a Berwald space or Douglas space.
In this paper, we mainly focus on m-th root metrics with weakly isotropic scalar curvature and obtain the following results:
Theorem 1.1. Let the m(≥3)-th root metric F be of weakly isotropic scalar curvature. Then its scalar curvature must vanish.
Let A:=ai1i2⋯im(x)yi1yi2⋯yim. If A=Fm is irreducible, then the further result is obtained as follows:
Theorem 1.2. Let F=m√A be the m(≥3)-th root metric. Assume that A is irreducible. Then the following are equivalent: (i) F is of weakly isotropic scalar curvature; (ii) its scalar curvature vanishes; (iii) it is Ricci-flat.
Based on Theorem 1.1, we obtain the result for locally conformally flat cubic Finsler metrics as following:
Theorem 1.3. Let F be a locally conformally flat cubic Finsler metric on a manifold M of dimension n(≥3). If F is of weakly isotropic scalar curvature, then F must be locally Minkowskian.
In this section, we mainly introduce several geometric quantities in Finsler geometry and several results that will be used later.
Let M be an n(≥3)-dimensional smooth manifold. The points in the tangent bundle TM are denoted by (x,y), where x∈M and y∈TxM. Let (xi,yi) be the local coordinates of TM with y=yi∂∂xi. A Finsler metric on M is a function F:TM⟶[0,+∞) such that
(1) F is smooth in TM∖{0};
(2) F(x,λy)=λF(x,y) for any λ>0;
(3) The fundamental quadratic form g=gij(x,y)dxi⊗dxj, where
gij(x,y)=[12F2(x,y)]yiyj |
is positively definite. We use the notations: Fyi:=∂F∂yi, Fxi:=∂F∂xi,F2yiyj:=∂2F2∂yi∂yj.
Let F be a Finsler metric on an n-dimensional manifold M, and let Gi be the geodesic coefficients of F, which are defined by
Gi:=14gij(F2xkyjyk−F2xj), |
where (gij)=(gij)−1. For any x∈M and y∈TxM∖{0}, the Riemann curvature Ry:=Rik(x,y)∂∂xi⊗dxk is defined by
Rik:=2Gixk−Gixjykyj+2GjGiyjyk−GiyjGjyk. |
The Ricci curvature Ric is the trace of the Riemann curvature defined by
Ric:=Rkk. |
The Ricci tensor is
Ricij:=12Ricyiyj. |
By the homogeneity of Ric, we have Ric=Ricijyiyj. The scalar curvature r of F is defined as
r:=gijRicij. | (2.1) |
A Finsler metric is said to be of weakly isotropic scalar curvature if there exists a 1-form θ=θi(x)yi and a scalar function χ=χ(x) such that
r=n(n−1)(θF+χ). | (2.2) |
An (α,β)-metric is a Finsler metric of the form
F=αϕ(s), |
where α=√aij(x)yiyj is a Riemannian metric, β=bi(x)yi is a 1-form, s:=βα and b:=∥β∥α<b0. It has been proved that F=αϕ(s) is a positive definite Finsler metric if and only if ϕ=ϕ(s) is a positive C∞ function on (−b0,b0) satisfying the following condition:
ϕ(s)−sϕ′(s)+(B−s2)ϕ″(s)>0,|s|≤b<b0, | (2.3) |
where B:=b2.
Let F=3√aijk(x)yiyjyk be a cubic metric on a manifold M of dimension n≥3. By choosing a suitable non-degenerate quadratic form α=√aij(x)yiyj and one-form β=bi(x)yi, it can be written in the form
F=3√pβα2+qβ3, |
where p and q are real constants such that p+qB≠0 (see [6]). The above equation can be rewritten as
F=α(ps+qs3)13, |
which means that F is also an (α,β)-metric with ϕ(s)=(ps+qs3)13. Then, by (2.3), we obtain
−p2B+p(4p+3qB)s2>0. | (2.4) |
Two Finsler metrics F and ˜F on a manifold M are said to be conformally related if there is a scalar function κ=κ(x) on M such that F=eκ(x)˜F. Particularly, an (α,β)-metric F=αϕ(βα) is said to be conformally related to a Finsler metric ˜F if F=eκ(x)˜F with ˜F=˜αϕ(˜s)=˜αϕ(˜β˜α). In the following, we always use symbols with a tilde to denote the corresponding quantities of the metric ˜F. Note that α=eκ(x)˜α, β=eκ(x)˜β, thus ˜s=s.
A Finsler metric that is conformally related to a locally Minkowski metric is said to be locally conformally flat. Thus, a locally conformally flat (α,β)-metric F has the form F=eκ(x)˜F, where ˜F=˜αϕ(˜β˜α) is a locally Minkowski metric.
Denoting
rij:=12(bi|j+bj|i),sij:=12(bi|j−bj|i),rij:=ailrlj,sij:=ailslj,rj:=birij,r:=biri,sj:=bisij,r00:=rijyiyj,si0:=sijyj,s0:=siyi, |
where bi:=aijbj, bi|j denotes the covariant differentiation with respect to α.
Let Gi and Giα denote the geodesic coefficients of F and α, respectively. The geodesic coefficients Gi of F=αϕ(βα) are related to Giα by
Gi=Giα+αQsi0+(−2Qαs0+r00)(Ψbi+Θα−1yi), |
where
Q:=ϕ′ϕ−sϕ′,Θ:=ϕϕ′−s(ϕϕ″+ϕ′ϕ′)2ϕ[(ϕ−sϕ)+(B−s2)ϕ″],Ψ:=ϕ″2[(ϕ−sϕ′)+(B−s2)ϕ″]. |
Assume that F=αϕ(βα) is conformally related to a Finsler metric ˜F=˜αϕ(˜β˜α) on M, i.e., F=eκ(x)˜F. Then
aij=e2κ(x)˜aij,bi=eκ(x)˜bi,˜b:=∥˜β∥˜α=√˜aij˜bi˜bj=b. |
Further, we have
bi|j=eκ(x)(˜bi∥j−˜bjκi+˜blκl˜aij),αΓlij=˜α˜Γlij+κjδli+κiδlj−κl˜aij,rij=eκ(x)˜rij+12eκ(x)(−˜bjκi−˜biκj+2˜blκl˜aij),sij=eκ(x)˜sij+12eκ(x)(˜biκj−˜bjκi),ri=˜ri+12(˜blκl˜bi−b2κi),r=e−κ(x)˜r,si=˜si+12(b2κi−˜blκl˜bi),rii=e−κ(x)˜rii+(n−1)e−κ(x)˜biκi,sji=e−κ(x)˜sji+12e−κ(x)(˜bjκi−˜biκj). |
Here ˜bi∥j denotes the covariant derivatives of ˜bi with respect to ˜α, αΓmij and ˜α˜Γmij denote Levi–Civita connections with respect to α and ˜α, respectively. In the following, we adopt the notations κi:=∂κ(x)∂xi, κij:=∂2κ(x)∂xi∂xj, κi:=˜aijκj, ˜bi:=˜aij˜bj, f:=˜biκi, f1:=κij˜biyj, f2:=κij˜bi˜bj, κ0:=κiyi, κ00:=κijyiyj and ∥▽κ∥2˜α:=˜aijκiκj.
Lemma 2.1. ([4]) Let F=eκ(x)˜F, where ˜F=˜αϕ(˜β˜α) is locally Minkowskian. Then the Ricci curvature of F is determined by
Ric=D1∥▽κ∥2˜α˜α+D2κ20+D3κ0f˜α+D4f2˜α2+D5f1˜α+D6˜α2+D7κ00, |
where Dk(k=1,...,7) is listed in Lemma 3.2 in [4].
Lemma 2.2. ([4]) Let F=eκ(x)˜F, where ˜F=˜αϕ(˜β˜α) is locally Minkowskian. Then the scalar curvature of F is determined by
r=12e−2κ(x)ρ−1[Σ1−(τ+ηλ2)Σ2−λη˜αΣ3−η˜α2Σ4], |
where
τ:=δ1+δB,η:=μ1+Y2μ,λ:=ε−δs1+δB,δ:=ρ0−ε2ρ2ρ,ε:=ρ1ρ2,μ:=ρ2ρ,Y:=√AijYiYj,Aij:=aij+δbibj,ρ:=ϕ(ϕ−sϕ′),ρ0:=ϕϕ″+ϕ′ϕ′,ρ1:=−s(ϕϕ″+ϕ′ϕ′)+ϕϕ′,ρ2:=s[s(ϕϕ″+ϕ′ϕ′)−ϕϕ′], |
and Σi(i=1,...,4) are listed in the proof of Lemma 3.3 in [4].
Lemma 2.3. ([14]) Let m-th root metric F=m√ai1i2⋯im(x)yi1yi2⋯yim be a Finsler metric on a manifold of dimension n. Then the Ricci curvature of F is a rational function in y.
In this section, we will prove the main theorems. Firstly, we give the proof of Theorem 1.1.
The proof of Theorem 1.1. For an m-th root metric F=m√ai1i2⋯im(x)yi1yi2⋯yim on a manifold M, the inverse of the fundamental tensor of F is given by (see [14])
gij=1(m−1)F2(AAij+(m−2)yiyj), | (3.1) |
where Aij=1m(m−1)∂2A∂yi∂yj and (Aij)=(Aij)−1. Thus, F2gij are rational functions in y.
By Lemma 2.3, the Ricci curvature Ric of m-th root metric is a rational function in y. Thus, Ricij:=Ricyiyj are rational functions. According to (2.1), we have
F2r=F2gijRicij. | (3.2) |
This means that F2r is a rational function in y.
On the other hand, if F is of weakly isotropic scalar curvature, according to (2.2), we obtain
F2r=n(n−1)(θF+χF2), |
where θ is a 1-form and χ is a scalar function. The right side of the above equation is an irrational function in y. Comparing it with (3.2), we have r=0.
In the following, the proof of Theorem 1.2 is given.
The proof of Theorem 1.2. By Theorem 1.1, we conclude that F is of weakly isotropic scalar curvature if and only if its scalar curvature vanishes. So we just need to prove that (ii) is equivalent to (iii). Assume that the scalar curvature vanishes. Hence, by (3.1), 0=r=gijRicij=1m−1F−2(AAij+(m−2)yiyj)Ricij holds. It means that
0=(AAij+(m−2)yiyj)Ricij=AAijRicij+(m−2)Ric. |
Since A is irreducible, Ric must be divided by A. Thus, Ric=0.
Conversely, if Ric=0, then by the definition of r we have r=0.
Based on Theorem 1.1, we can prove Theorem 1.3 for locally conformally flat cubic metrics.
The proof of Theorem 1.3. Assume that the locally conformally flat cubic metric F is of weakly isotropic scalar curvature. Then, by Lemma 2.2 and Theorem 1.1, we obtain the scalar curvature vanishes, i.e.,
Σ1−(τ+ηλ2)Σ2−λη˜αΣ3−η˜α2Σ4=0. |
Further, by detailed expressions of Σi(i=1,⋯4), the above equation can be rewritten as
B(4p+3qB)κ20−4(4p+3qB)˜βκ0f+4p˜α2f2(4p+3qB)8˜α2s2γ7+Tγ6=0, | (3.3) |
where γ:=pB˜α2−(4p+3qB)˜β2 and T has no γ−1.
Thus, the first term of (3.3) can be divided by γ. It means that there is a function h(x) on M such that
B(4p+3qB)κ20−4(4p+3qB)˜βκ0f+4p˜α2f2=h(x)γ. |
The above equation can be rewritten as
B(4p+3qB)κ20−4(4p+3qB)˜βκ0f+4p˜α2f2=h(x)[pB˜α2−(4p+3qB)˜β2]. | (3.4) |
Differentiating (3.4) with yi yields
B(4p+3qB)κ0κi−2(4p+3qB)(˜biκ0+˜βκi)f+4p˜ailylf2=h(x)[pB˜ailyl−(4p+3qB)˜β˜bi]. | (3.5) |
Differentiating (3.5) with yj yields
B(4p+3qB)κiκj−2(4p+3qB)(˜biκj+˜bjκi)f+4p˜aijf2=h(x)[pB˜aij−(4p+3qB)˜bi˜bj]. |
Contracting the above with ˜bi˜bj yields
Bf2(8p+9qB)=3B2h(x)(p+qB). |
Thus, we have
h(x)=(8p+9qB)f23B(p+qB). | (3.6) |
Substituting (3.6) into (3.5) and contracting (3.5) with ˜bi yield
(4p+3qB)f(f˜β−Bκ0)=0. | (3.7) |
Furthermore, by (2.4) and 4p+3qB≠0, we have f(f˜β−Bκ0)=0.
Case I: f=0. It means h(x)=0 by (3.6). Thus, one has that κi=0 by (3.4), which means
κ=constant. |
Case II: f≠0. It implies that f˜β−Bκ0=0. Substituting it into (3.4), we obtain
˜β2=−B˜α2, |
which does not exist.
Above all, we have κ=constant. Thus we conclude that the conformal transformation must be homothetic.
X. Zhang: Conceptualization, Validation, Formal analysis, Resources, Software, Investigation, Methodology, Supervision, Writing-original draft, Writing-review and editing, Project administration, Funding acquisition; C. Ma: Formal analysis, Software, Investigation, Supervision, Writing-review and editing; L. Zhao: Resources, Investigation, Supervision. All authors have read and agreed to the published version of the manuscript.
The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.
Xiaoling Zhang's research is supported by National Natural Science Foundation of China (No. 11961061, 11761069).
The authors declare that they have no conflicts of interest.
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