arXiv subjects

Ahmad El Soufi

Publications and source records attributed to Ahmad El Soufi.

26 records · Page 2Linked to original sources

arXiv2007

Extremal $G$-invariant eigenvalues of the Laplacian of $G$-invariant metrics

The study of extremal properties of the spectrum often involves restricting the metrics under consideration. Motivated by the work of Abreu and Freitas in the case of the sphere $S^2$ endowed with $S^1$-invariant metrics, we consider the subsequence $λ_k^G$ of the spectrum of a Riemannian manifold $M$ which corresponds to metrics and functions invariant under the action of a compact Lie group $G$. If $G$ has dimension at least 1, we show that the functional $λ_k^G$ admits no extremal metric under volume-preserving $G$-invariant deformations. If, moreover, $M$ has dimension at least three, then the functional $λ_k^G$ is unbounded when restricted to any conformal class of $G$-invariant metrics of fixed volume. As a special case of this, we can consider the standard O(n)-action on $S^n$; however, if we also require the metric to be induced by an embedding of $S^n$ in $\mathbb{R}^{n+1}$, we get an optimal upper bound on $λ_k^G$.

math.DG↗

arXiv2007

A unique extremal metric for the least eigenvalue of the Laplacian on the Klein bottle

We prove the following conjecture recently formulated by Jakobson, Nadirashvili and Polterovich \cite{JNP}: on the Klein bottle $\mathbb{K}$, the metric of revolution $$g_0= {9+ (1+8\cos ^2v)^2\over 1+8\cos ^2v} (du^2 + {dv^2\over 1+8\cos ^2v}),$$ $0\le u <\fracπ2$, $0\le v <π$, is the \emph{unique} extremal metric of the first eigenvalue of the Laplacian viewed as a functional on the space of all Riemannian metrics of given area. The proof leads us to study a Hamiltonian dynamical system which turns out to be completely integrable by quadratures.

math.MG↗

arXiv2007

A Gamma-convergence argument for the blow-up of a non-local semilinear parabolic equation with Neumann boundary conditions

In this paper we study a simple non-local semilinear parabolic equation with Neumann boundary condition. We give local existence result and prove global existence for small initial data. A natural non increasing in time energy is associated to this equation. We prove that the solution blows up at finite time $T$ if and only if its energy is negative at some time before $T$. The proof of this result is based on a Gamma-convergence technique.

math.AP↗

arXiv2007

Laplacian eigenvalues functionals and metric deformations on compact manifolds

In this paper, we investigate critical points of the Laplacian's eigenvalues considered as functionals on the space of Riemmannian metrics or a conformal class of metrics on a compact manifold. We obtain necessary and sufficient conditions for a metric to be a critical point of such a functional. We derive specific consequences concerning possible locally maximizing metrics. We also characterize critical metrics of the ratio of two consecutive eigenvalues.

math.MG↗

arXiv2005

Greatest least eigenvalue of the Laplacian on the Klein bottle

We prove the following conjecture recently formulated by Jakobson, Nadirashvili and Polterovich \cite{JNP}: For any Riemannian metric $g$ on the Klein bottle $\mathbb{K}$ one has $$λ\_1 (\mathbb{K}, g) A (\mathbb{K}, g)\le 12 πE(2\sqrt 2/3),$$ where $λ\_1(\mathbb{K},g)$ and $A(\mathbb{K},g)$ stand for the least positive eigenvalue of the Laplacian and the area of $(\mathbb{K},g)$, respectively, and $E$ is the complete elliptic integral of the second kind. Moreover, the equality is uniquely achieved, up to dilatations, by the metric $$g\_0= {9+ (1+8\cos ^2v)^2\over 1+8\cos^2v} (du^2 + {dv^2\over 1+8\cos ^2v}),$$ with $0\le u,v <π$. The proof of this theorem leads us to study a Hamiltonian dynamical system which turns out to be completely integrable by quadratures.

math.MG↗

arXiv2005

Critical potentials of the eigenvalues and eigenvalue gaps of Schrödinger operators

Let $M$ be a compact Riemannian manifold with or without boundary, and let $-Δ$ be its Laplace-Beltrami operator. For any bounded scalar potential $q$, we denote by $λ\_i(q)$ the $i$-th eigenvalue of the Schrödinger type operator $-Δ+ q$ acting on functions with Dirichlet or Neumann boundary conditions in case $\partial M \neq \emptyset$. We investigate critical potentials of the eigenvalues $λ\_i$ and the eigenvalue gaps $G\_{ij}=λ\_j -λ\_i$ considered as functionals on the set of bounded potentials having a given mean value on $M$. We give necessary and sufficient conditions for a potential $q$ to be critical or to be a local minimizer or a local maximizer of these functionals. For instance, we prove that a potential $q \in L^\infty (M)$ is critical for the functional $λ\_2$ if and only if, $q$ is smooth, $λ\_2(q)=λ\_3(q)$ and there exist second eigenfunctions $f\_1 ,...,f\_k$ of $-Δ+ q$ such that $Σ\_j f\_j^2 = 1$. In particular, $λ\_2$ (as well as any $λ\_i$) admits no critical potentials under Dirichlet Boundary conditions. Moreover, the functional $λ\_2$ never admits locally minimizing potentials.

math.MG↗

arXiv2004

Eigenvalues of the Laplacian acting on $p$-forms and metric conformal deformations

Let $(M,g)$ be a compact connected orientable Riemannian manifold of dimension $n\ge4$ and let $λ_{k,p} (g)$ be the $k$-th positive eigenvalue of the Laplacian $Δ_{g,p}=dd^*+d^*d$ acting on differential forms of degree $p$ on $M$. We prove that the metric $g$ can be conformally deformed to a metric $g'$, having the same volume as $g$, with arbitrarily large $λ_{1,p} (g')$ for all $p\in[2,n-2]$. Note that for the other values of $p$, that is $p=0, 1, n-1$ and $n$, one can deduce from the literature that, $\forall k >0$, the $k$-th eigenvalue $λ_{k,p}$ is uniformly bounded on any conformal class of metrics of fixed volume on $M$. For $p=1$, we show that, for any positive integer $N$, there exists a metric $g_N$ conformal to $g$ such that, $\forall k\le N$, $λ_{k,1} (g_N) =λ_{k,0} (g_N) $, that is, the first $N$ eigenforms of $Δ_{g_N,1}$ are all exact forms.

math.DG↗

arXiv2004

Extremal eigenvalues of the Laplacian in a conformal class of metrics : the "conformal spectrum"

Let $M$ be a compact connected manifold of dimension $n$ endowed with a conformal class $C$ of Riemannian metrics of volume one. For any integer $k\geq0$, we consider the conformal invariant $λ_k ^c (C)$ defined as the supremum of the $k$-th eigenvalue $λ_k (g)$ of the Laplace-Beltrami operator $Δ_g$, where $g$ runs over $C$. First, we give a sharp universal lower bound for $λ_k ^c (C)$ extending to all $k$ a result obtained by Friedlander and Nadirashvili for $k=1$. Then, we show that the sequence $ \{λ_k ^c (C) \}$, that we call "conformal spectrum", is strictly increasing and satisfies, $\forall k\geq 0$, $λ_{k+1} ^c (C)^{n/2} - λ_k ^c (C)^{n/2} \geq n^{n/2} ω_n $, where $ω_n $ is the volume of the $n$-dimensional standard sphere. When $M$ is an orientable surface of genus $γ$, we also consider the supremum $λ_k ^{top} (γ)$ of $λ_k(g)$ over the set of all the area one Riemannian metrics on $M$, and study the behavior of $λ_k ^{top} (γ)$ in terms of $γ$.

math.DG↗