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Ludovick Gagnon

Publications and source records attributed to Ludovick Gagnon.

12 recordsLinked to original sources
arXiv2026

Quantitative Fredholm backstepping and rapid stabilization

In this paper, we address the existence of Fredholm backstepping transformations for self-adjoint and skew-adjoint operators $A$. Under suitable assumptions on the operator $A$ and the possibly unbounded control operator $B$, we prove the existence of a Fredholm backstepping transformation for operators of order strictly greater than $1$. This work overcomes two major limitations of the classical Fredholm backstepping framework. One of the main contributions is the explicit identification of the underlying isomorphism used in the construction of the transformation $T$, thereby bypassing the compactness arguments and Riesz basis mechanisms traditionally used in the literature. This explicit structure enables us to derive quantitative and sharp estimates for $\|T\|_{\mathcal{L}(H;H)}$ and $\|T^{-1}\|_{\mathcal{L}(H;H)}$ with respect to the decay rate $λ$. As a consequence, we obtain quantitative rapid stabilization and small-time null controllability results for a broad class of operators.

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arXiv2026

DeepRitzSplit Neural Operator for Phase-Field Models via Energy Splitting

The multi-scale and non-linear nature of phase-field models of solidification requires fine spatial and temporal discretization, leading to long computation times. This could be overcome with artificial-intelligence approaches. Surrogate models based on neural operators could have a lower computational cost than conventional numerical discretization methods. We propose a new neural operator approach that bridges classical convex-concave splitting schemes with physics-informed learning to accelerate the simulation of phase-field models. It consists of a Deep Ritz method, where a neural operator is trained to approximate a variational formulation of the phase-field model. By training the neural operator with an energy-splitting variational formulation, we enforce the energy dissipation property of the underlying models. We further introduce a custom Reaction-Diffusion Neural Operator (RDNO) architecture, adapted to the operators of the model equations. We successfully apply the deep learning approach to the isotropic Allen-Cahn equation and to anisotropic dendritic growth simulation. We demonstrate that our physically-informed training provides better generalization in out-of-distribution evaluations than data-driven training, while achieving faster inference than traditional Fourier spectral methods.

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arXiv2025

Control of blow-up profiles for the mass-critical focusing nonlinear Schrödinger equation on bounded domains

In this paper, we consider the mass-critical focusing nonlinear Schrödinger on bounded two-dimensional domains with Dirichlet boundary conditions. In the absence of control, it is well-known that free solutions starting from initial data sufficiently large can blow-up. More precisely, given a finite number of points, there exists particular profiles blowing up exactly at these points at the blow-up time. For pertubations of these profiles, we show that, with the help of an appropriate nonlinear feedback law located in an open set containing the blow-up points, the blow-up can be prevented from happening. More specifically, we construct a small-time control acting just before the blow-up time. The solution may then be extended globally in time. This is the first result of control for blow-up profiles for nonlinear Schrödinger type equations. Assuming further a geometrical control condition on the support of the control, we are able to prove a null-controllability result for such blow-up profiles. Finally, we discuss possible extensions to three-dimensional domains.

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arXiv2024

Fredholm backstepping for critical operators and application to rapid stabilization for the linearized water waves

Fredholm-type backstepping transformation, introduced by Coron and Lü, has become a powerful tool for rapid stabilization with fast development over the last decade. Its strength lies in its systematic approach, allowing to deduce rapid stabilization from approximate controllability. But limitations with the current approach exist for operators of the form $|D_x|^α$ for $α\in (1,3/2]$. We present here a new compactness/duality method which hinges on Fredholm's alternative to overcome the $α=3/2$ threshold. More precisely, the compactness/duality method allows to prove the existence of a Riesz basis for the backstepping transformation for skew-adjoint operator verifying $α>1$, a key step in the construction of the Fredholm backstepping transformation, where the usual methods only work for $α>3/2$. The illustration of this new method is shown on the rapid stabilization of the linearized capillary-gravity water wave equation exhibiting an operator of critical order $α=3/2$.

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arXiv2021

Fredholm transformation on Laplacian and rapid stabilization for the heat equation

We study the rapid stabilization of the heat equation on the 1-dimensional torus using the backstepping method with a Fredholm transformation. We prove that, under some assumption on the control operator, two scalar controls are necessary and sufficient to get controllability and rapid stabilization. This classical framework allows us to present the backstepping method with Fredholm transformations on Laplace operators in a sharp functional setting, which is the main objective of this work. Finally, we prove that the same Fredholm transformation also leads to the local rapid stability of the viscous Burgers equation.

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arXiv2020

A Fredholm transformation for the rapid stabilization of a degenerate parabolic equation

This paper deals with the rapid stabilization of a degenerate parabolic equation with a right Dirich-let control. Our strategy consists in applying a backstepping strategy, which seeks to find an invertible transformation mapping the degenerate parabolic equation to stabilize into an exponentially stable system whose decay rate is known and as large as we desire. The transformation under consideration in this paper is Fredholm. It involves a kernel solving itself another PDE, at least formally. The main goal of the paper is to prove that the Fredholm transformation is well-defined, continuous and invertible in the natural energy space. It allows us to deduce the rapid stabilization.

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arXiv2017

Sufficient Conditions for the Controllability of Wave Equations with a Transmission Condition at the Interface

We consider waves travelling in two different mediums each endowed with a different constant speed of propagation. At the interface between the two mediums, the refraction of the rays of the optic geometry is described by the Snell-Descartes law. We provide sufficient conditions on the geometry of the mediums and on the speed of propagation for the boundary controllability.

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arXiv2016

Recovering the uniform boundary observability with spectral Legendre-Galerkin formulations of the 1-D wave equation

For a Legendre-Galerkin semi-discretization of the 1-D homogeneous wave equation, the high frequency components of the numerical solution prevent us from obtaining the boundary observability (inequality), uniformly with regard to the discretization parameter. A classical Fourier filtering that filters out the high frequencies is sufficient to recover the uniform observability. Unfortunately, this remedy needs to compute all the frequencies of the underlying system. In this paper we present three cheaper alternative remedies, namely a spectral filtering, a mixed formulation and Nitsche's method to append Dirichlet type boundary conditions. Our numerical results show indeed that uniform boundary observability inequalities may be recovered. On another hand, surprisingly, none of them seems to provide a uniform direct (or trace) inequality, a property which is needed in some existing general convergence results for the adjoint boundary controllability problem. We finally show in a numerical example that, despite this fact, convergence of the control approximations holds whenever the uniform observability inequality is observed numerically.

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arXiv2016

Qualitative Description of the Particle Trajectories for the $N$-Solitons Solution of the Korteweg-de Vries Equation

The qualitative properties of the particle trajectories of the $N$-solitons solution of the KdV equation are recovered from the first order velocity field by the introduction of the stream function. Numerical simulations show an accurate depth dependance of the particles trajectories for solitary waves. Failure of the free surface kinematic boundary condition for the first order type velocity field is highlighted.

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arXiv2016

Rapid Stabilization of a Linearized Bilinear $1-D$ Schrödinger Equation

We consider the one dimensional Schrödinger equation with a bilinear control and prove the rapid stabilization of the linearized equation around the ground state. The feedback law ensuring the rapid stabilization is obtained using a transformation mapping the solution to the linearized equation on the solution to an exponentially stable target linear equation. A suitable condition is imposed on the transformation in order to cancel the non-local terms arising in the kernel system. This conditions also insures the uniqueness of the transformation. The continuity and invertibility of the transformation follows from exact controllability of the linearized system.

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arXiv2016

Lagrangian Controllability of the 1-D Korteweg-de Vries Equation

We consider in this paper the problem of the Lagrangian controllability for the Korteweg-de Vries equation. Using the $N$-solitons solution, we prove that, for any length of the spatial domain $L>0$ and any time $T>0$, it is possible to choose appropriate boundary controls of KdV equation such that the flow associated to this solution exit the domain in time $T$.

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