∙ A minimax method for the spectral fractional Laplacian and related evolution problems ∙
ArXiv, 2025. Submitted for publication.

José A. Carrillo , Yuji Nakatsukasa , Endre Süli

We present a numerical method for the approximation of the inverse of the fractional Laplacian (−Δ)s, based on its spectral definition, using rational functions to approximate the fractional power of a matrix. The proposed numerical method is fast and accurate, benefiting from the fact that the matrix arises from a finite element approximation of the Laplacian, which makes it applicable to a wide range of domains with potentially irregular shapes. We make use of state-of-the-art software to compute the best rational approximation of a fractional power. We analyze the convergence rate of our method and validate our findings through a series of numerical experiments. Additionally, we apply the proposed numerical method to different evolution problems that involve the fractional Laplacian through an interaction potential: the fractional porous medium equation and the fractional Keller-Segel equation. We then investigate the accuracy of the resulting numerical method, focusing in particular on the accurate reproduction of qualitative properties of the associated analytical solutions to these partial differential equations.

∙ Rate of Convergence for a Nonlocal-to-local Limit in One Dimension ∙
ArXiv, 2025. Submitted for publication

José A. Carrillo , Charles Elbar , Jakub Skrzeczkowski

We consider a nonlocal approximation of the quadratic porous medium equation where the pressure is given by a convolution with a mollification kernel. It is known that when the kernel concentrates around the origin, the nonlocal equation converges to the local one. In one spatial dimension, for a particular choice of the kernel, and under mere assumptions on the initial condition, we quantify the rate of convergence in the 2-Wasserstein distance. Our proof is very simple, exploiting the so-called Evolutionary Variational Inequality for both the nonlocal and local equations as well as a priori estimates. We also present numerical simulations using the finite volume method, which suggests that the obtained rate can be improved - this will be addressed in a forthcoming work.

∙ Finite Element Approximation of the Fractional Porous Medium Equation ∙
Numerische Mathematik, 2025.

José A. Carrillo , Endre Süli

We construct a finite element method for the numerical solution of a fractional porous medium equation on a bounded open Lipschitz polytopal domains. The pressure in the model is defined as the solution of a fractional Poisson equation, involving the fractional Neumann Laplacian in terms of its spectral definition. We perform a rigorous passage to the limit as the spatial and temporal discretization parameters tend to zero and show that a subsequence of the sequence of finite element approximations defined by the proposed numerical method converges to a bounded and nonnegative weak solution of the initial-boundary-value problem under consideration. This result can be therefore viewed as a constructive proof of the existence of a nonnegative, energy-dissipative, weak solution to the initial-boundary-value problem for the fractional porous medium equation under consideration, based on the Neumann Laplacian. The convergence proof relies on results concerning the finite element approximation of the spectral fractional Laplacian and compactness techniques for nonlinear partial differential equations, together with properties of the equation, which are shown to be inherited by the numerical method. We also prove that the total energy associated with the problem under consideration exhibits exponential decay in time.

∙ A finite-volume scheme for fractional diffusion on bounded domains ∙
European Journal of Applied Mathematics, 1-21, 2024.

Rafael Bailo , José A. Carrillo , David Gómez Castro

We propose a new fractional Laplacian for bounded domains, expressed as a conservation law and thus particularly suited to finite-volume schemes. Our approach permits the direct prescription of no-flux boundary conditions. We first show the well-posedness theory for the fractional heat equation. We also develop a numerical scheme, which correctly captures the action of the fractional Laplacian and its anomalous diffusion effect. We benchmark numerical solutions for the L'evy-Fokker-Planck equation against known analytical solutions. We conclude by numerically exploring properties of these equations with respect to their stationary states and long-time asymptotics.