arXiv subjects

Fabrice Bethuel

Publications and source records attributed to Fabrice Bethuel.

10 recordsLinked to original sources

arXiv2020

Asymptotics for two-dimensional vectorial Allen-Cahn systems

The formation of codimension-one interfaces for multi-well gradient-driven problems is well-known and established in the scalar case, where the equation is often referred to as the Allen-Cahn equation. The proofs rely for a large on a monotonicity formula for the energy density, which is itself related to the vanishing of the so-called discrepancy function. The vectorial case in contrast is quite open. This lack of results and insight is to a large extend related to the absence of known appropriate monotonicity formula. In this paper, we focus on the \emph{elliptic case in two dimensions}, and introduce methods, relying on the analysis of the partial differential equation, which allow to circumvent the lack of monotonicity formula for the energy density. In the last part of the paper, we recover a \emph{new monotonicity formula} which relies on a \emph{new discrepancy relation}. These tools allow to extend to the vectorial case in two dimensions most of the results obtained for the scalar case. We emphasize also some \emph{specific features} of the vectorial case.

math.AP↗

arXiv2018

A counterexample to the weak density of smooth maps between manifolds in Sobolev spaces

The present paper presents a counterexample to the sequentially weak density of smooth maps between two manifolds $M$ and $N$ in the Sobolev space $W^{1, p} (M, N)$, in the case $p$ is an integer. It has been shown that, if $p<\dim M $ is not an integer and the $[p]$-th homotopy group $π_{[p]}(N)$ of $N$ is not trivial, $[p]$ denoting the largest integer less then $p$, then smooth maps are not sequentially weakly dense in $W^{1, p} (M, N)$ for the strong convergence. On the other, in the case $p< \dim M$ is an integer, examples have been provided where smooth maps are actually sequentially weakly dense in $W^{1, p} (M, N)$ with $π_{p}(N)\not = 0$. This is the case for instance for $M= \mathbb B^m$, the standard ball in $\mathbb R^m$, and $N=\mathbb S^p$ the standard sphere of dimension $p$, for which $π_{p}(N) =\mathbb Z$. The main result of this paper shows however that such a property does not holds for arbitrary manifolds $N$ and integers $p$.Our counterexample deals with the case $p=3$, $\dim M\geq 4$ and $N=\mathbb S^2$, for which the homotopy group $π_3(\mathbb S^2)=\mathbb Z$ is related to the Hopf fibration.

math.FA↗

arXiv2018

Concentration sets for multiple equal-depth wells potentials in the 2D elliptic case

The formation of codimension-one interfaces for multiwell gradient-driven problems is well-known and established in the scalar case, where the equation is often referred to as the Allen-Cahn equation. The vectorial case in contrast is quite open. This lack of results and insight is to a large extend related to the absence of known appropriate monotonicity formula. In this paper, we focus on the elliptic case in two dimensions, and introduce some methods which allow to circumvent the lack of monotonicity formula. This methods lead, as expected, to concentration on one-dimensional rectifiable sets.

math.AP↗

arXiv2014

A new obstruction to the extension problem for Sobolev maps between manifolds

The main result of the present paper, combined with earlier results of Hardt and Lin settles the extension problem for $W^{1,p}(\mathcal M, \mathcal N)$, where $\mathcal M$ and $\mathcal N$ are compact riemannian manfolds, $\mathcal M$ having non-empty smooth boundary and assuming moreover that $\mathcal N$ is simply connected. The main question which is studied is the following: Given a map in the trace space $W^{1-\frac{1}{p}, p} (\partial \mathcal M, \mathcal N)$, does it possess an extension in $W^{1,p}(\mathcal M, \mathcal N)$? We show that the answer is negative in the case $\mathfrak p_{c} +1\leq p<m={\rm dim} \, \mathcal M$, where the number $\mathfrak p_{c}$ is related to the topology of $\mathcal N$. We also adress the case $\mathcal N$ is not simply connected, providing various results and rising some open questions. In particular, we stress in that case the relationship between the extension problem and the lifting problem to the universal covering manifold.

math.FA↗

arXiv2013

The motion law of fronts for scalar reaction-diffusion equations with multiple wells : the degenerate case,

We derive a precise motion law for fronts of solutions to scalar one-dimensional reaction-diffusion equations with equal depth multiple-wells, in the case the second derivative of the potential vanishes at its minimizers. We show that, renormalizing time in an algebraic way, the motion of fronts is governed by a simple system of ordinary differential equations of nearest neighbor interaction type. These interactions may be either attractive or repulsive. Our results are not constrained by the possible occurrence of collisions nor splittings. They present substantial differences with the results obtained in the case the second derivative does not vanish at the wells, a case which has been extensively studied in the literature, and where fronts have been showed to move at exponentially small speed, with motion laws which are not renormalizable.

math.AP↗

arXiv2009

On the Korteweg-de Vries long-wave approximation of the Gross-Pitaevskii equation II

In this paper, we proceed along our analysis of the Korteweg-de Vries approximation of the Gross-Pitaevskii equation initiated in a previous paper. At the long-wave limit, we establish that solutions of small amplitude to the one-dimensional Gross-Pitaevskii equation split into two waves with opposite constant speeds $\pm \sqrt{2}$, each of which are solutions to a Korteweg-de Vries equation. We also compute an estimate of the error term which is somewhat optimal as long as travelling waves are considered. At the cost of higher regularity of the initial data, this improves our previous estimate.

math.AP↗

arXiv2009

On the Korteweg-de Vries long-wave approximation of the Gross-Pitaevskii equation I

The fact that the Korteweg-de-Vries equation offers a good approximation of long-wave solutions of small amplitude to the one-dimensional Gross-Pitaevskii equation was derived several years ago in the physical literature. In this paper, we provide a rigorous proof of this fact, and compute a precise estimate for the error term. Our proof relies on the integrability of both the equations. In particular, we give a relation between the invariants of the two equations, which, we hope, is of independent interest.

math.AP↗

arXiv2009

Slow motion for gradient systems with equal depth multiple-well potentials

For scalar reaction-diffusion in one space dimension, it is known for a long time that fronts move with an exponentially small speed for potentials with several distinct mini- mizers. The purpose of this paper is to provide a similar result in the case of systems. Our method relies on a careful study of the evolution of localized energy. This approach has the advantage to relax the preparedness assumptions on the initial datum.

math.AP↗

arXiv2008

On the linear wave regime of the Gross-Pitaevskii equation

We study a long wave-length asymptotics for the Gross-Pitaevskii equation corresponding to perturbation of a constant state of modulus one. We exhibit lower bounds on the first occurence of possible zeros (vortices) and compare the solutions with the corresponding solutions to the linear wave equation or variants. The results rely on the use of the Madelung transform, which yields the hydrodynamical form of the Gross-Pitaevskii equation, as well as of an augmented system.

math.AP↗

arXiv2008

Travelling waves for the Gross-Pitaevskii equation II

The purpose of this paper is to provide a rigorous mathematical proof of the existence of travelling wave solutions to the Gross-Pitaevskii equation in dimensions two and three. Our arguments, based on minimization under constraints, yield a full branch of solutions, and extend earlier results, where only a part of the branch was built. In dimension three, we also show that there are no travelling wave solutions of small energy.

math.AP↗