arXiv subjects

Sidy M. Djitte

Publications and source records attributed to Sidy M. Djitte.

8 recordsLinked to original sources

arXiv2026

Pohozaev-like identity for the regional fractional laplacian

We establish a new integration by parts formula for the regional fractional laplacian $(-Δ)^s_Ω$ in bounded open sets of class $C^2$. As a direct application, we prove that weak solutions to the corresponding Dirichlet problem satisfy a Pohozaev-like identity with an explicit remainder term. We apply the later to eigenvalue problems in the unit ball and discuss its potential use in establishing boundary-type unique continuation properties.

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arXiv2026

A Lions' type formula for some reproducing kernel Hilbert spaces of fractional harmonic functions

In \cite{Lions}, J. L. Lions considered a reproducing kernel Hilbert space (RKHS) of harmonic functions on a regular domain with Sobolev traces and obtained a formula that expresses the kernel of this space as an integral on the boundary of some derivatives of the Green function associated with the Laplace operator and the homogeneous Dirichlet boundary condition. This result was simplified and extended later by Englis, Lukkassen, Peetre, and Persson in \cite{ELPL} to more general elliptic systems of even orders. In particular, they emphasized that the resemblance between Lions' type formula and the Hadamard variational formula only appears when the operator is of order $2$. In this paper, we investigate some RKHS of $a$-harmonic functions, where $a$ in $(0,1)$ refers to a fractional exponent of the Laplace operator. For such fractional order pseudo-differential operators, the local nonhomogeneous Dirichlet problem can be addressed by means of some $a$-transmission Sobolev spaces, which were introduced by Hörmander in the sixties and recently developed by Grubb in a series of papers. We deduce from these works a fractional Poisson formula, which is applied to obtain a Lions' type formula. We observe, in particular, that despite the order of the operator not being $2$, this formula resembles the Hadamard variational formula that we prove in the companion paper \cite{sidy-franck_1}. As a complementary remark, we observe that for a family of RKHS associated with the steady Stokes system, a second order system, there is also a Lions' type formula for their two-point kernels, which turn out not to be similar to the corresponding Hadamard variation formula.

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arXiv2026

Pointwise Hadamard variational formula for the fractional Laplacian

We establish pointwise formulas for the shape derivative of solutions to the Dirichlet problem associated with the fractional Laplacian. Specifically, we consider the equation $(-Δ)^s u = h$ in $Ω$ and $u=0$ in $Ω^c$, where the right-hand side $h$ is either a Dirac delta distribution or a Lipschitz function. In both cases, we prove that the corresponding solution is shape differentiable in every direction and we derive a formula for the pointwise value of its shape derivative. These formulas involve integral on the domain's boundary and fractional Neumann's traces. This extends to the case of the fractional Laplacian the well-known Hadamard variational formula for the standard Laplacian. Our argument is in the spirit of \cite{Ushikoshi, Kozono-Ushikoshi} and is based on PDEs techniques.

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arXiv2026

A Brezis and Peletier type result for the fractional Robin function

This paper is devoted to the Laplacian operator of fractional order $s\in (0,1)$ in several dimensions. We consider the equation $(-Δ)^su=f(x,u)$ in $Ω$, $u=0$ in $Ω^c$ and establish a representation formula for partial derivatives of solutions in terms of the normal derivative $u/δ^s$. As a consequence, we prove that solutions to the overdetermined problem $(-Δ)^su=f(x,u)$ in $Ω$, $u=0$ in $Ω^c$, and $u/δ^s=0$ on $\partialΩ$ are globally Lipschitz continuous provided that $2s>1$. We also prove a Pohozaev-type identity for the Green function and, in particular, obtain a formula for the gradient of the Robin function, which extends to the fractional setting some results obtained by Brézis and Peletier in \cite{Bresiz} in the classical case of the Laplacian. Finally, an application to the nondegeneracy of critical points of the fractional Robin function in symmetric domains is discussed.

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arXiv2025

Shape derivative approach to fractional overdetermined problems

We use shape derivative approach to prove that balls are the only convex and $C^{1,1}$ regular domains in which the fractional overdetermined problem \begin{equation*} \left\{\begin{aligned} \Ds u&= λ_{s, p} u^{p-1}\quad\text{in}\quad\Om \\ u &= 0\quad \text{in}\quad\R^N\setminus \Om\\ u/d^s&=C_0\quad\text{on\;\; $\partialØ$} \end{aligned} \right. \end{equation*} admits a nontrivial solution for $p\in [1, 2]$ and where $λ_{s, p}= λ_{s, p}(Ø)$ is the best constant in the family of Subcritical Sobolev inequalities. In the cases $p=1$ and $p=2$, we recover the classical symmetry results of Serrin, corresponding to the torsion problem and the first Dirichlet eigenvalue problem, respectively (see \cite{FS-15}). We note that for $p\in (1,2)$, the above problem lies outside the framework of \cite{FS-15}, and the methods developed therein do not apply. Our approach extends to the fractional setting a method initially developed by A. Henrot and T. Chatelain in \cite{CH-99}, and relies on the use of domain derivatives combined with the continuous Steiner symmetrization introduced by Brock in \cite{Brock-00}.

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arXiv2023

A few representation formulas for solutions of fractional Laplace equations

This paper is devoted to the Laplacian operator of fractional order $s\in (0,1)$ in several dimensions. We first establish a representation formula for the partial derivatives of the solutions of the homogeneous Dirichlet problem. Along the way, we obtain a Pohozaev-type identity for the fractional Green function and of the fractional Robin function. The latter extends to the fractional setting a formula obtained by Brézis and Peletier, see \cite{Bresiz}, in the classical case of the Laplacian. As an application we consider the particle system extending the classical point vortex system to the case of a fractional Laplacian. We observe that, for a single particle in a bounded domain, the properties of the fractional Robin function are crucial for the study of the steady states. We also extend the classical Hadamard variational formula to the fractional Green function as well as to the shape derivative of weak solution to the homogeneous Dirichlet problem. Finally we turn to the in homogeneous Dirichlet problem and extend a formula by J.L. Lions, see \cite{Lions}, regarding the kernel of the reproducing kernel Hilbert space of harmonic functions to the case of $s$-harmonic functions. We observe that, despite the order of the operator is not $2$, this formula looks like the Hadamard variational formula, answering in a negative way to an open question raised in \cite{ELPL}.

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arXiv2022

Nonradiality of second fractional eigenfunctions of thin annuli

In the present paper, we study properties of the second Dirichlet eigenvalue of the fractional Laplacian of annuli-like domains and the corresponding eigenfunctions. In the first part, we consider an annulus with inner radius $R$ and outer radius $R+1$. We show that for $R$ sufficiently large any corresponding second eigenfunction of this annulus is nonradial. In the second part, we investigate the second eigenvalue in domains of the form $B_1(0)\setminus \overline{B_τ(a)}$, where $a$ is in the unitary ball and $0<τ<1-|a|$. We show that this value is maximized for $a=0$, if the set $B_1(0)\setminus \overline{B_τ(0)}$ has no radial second eigenfunction. We emphasize that the first part of our paper implies that this assumption is indeed nonempty.

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arXiv2021

Symmetry of odd solutions to equations with fractional Laplacian

We present a symmetry result to solutions of equations involving the fractional Laplacian in a domain with at least two perpendicular symmetries. We show that if the solution is continuous, bounded, and odd in one direction such that it has a fixed sign on one side, then it will be symmetric in the perpendicular direction. Moreover, the solution will be monotonic in the part where it is of fixed sign. In addition, we present also a class of examples in which our result can be applied.

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