Fractional Elliptic Problems on Lipschitz Domains: Regularity and Approximation

TL;DR

This survey explores the regularity and approximation of fractional elliptic problems on Lipschitz domains, presenting new regularity results, error estimates for finite element discretizations, and robust preconditioning techniques.

Contribution

It introduces novel optimal shift theorems in Besov spaces, extends regularity results to fractional p-Laplacian, and improves error estimates for finite element methods on graded meshes.

Findings

New regularity results in Besov and Sobolev spaces

Enhanced error estimates for finite element discretizations

Robust BPX preconditioner for fractional problems

Abstract

This survey hinges on the interplay between regularity and approximation for linear and quasi-linear fractional elliptic problems on Lipschitz domains. For the linear Dirichlet integral Laplacian, after briefly recalling H\"older regularity and applications, we discuss novel optimal shift theorems in Besov spaces and their Sobolev counterparts. These results extend to problems with finite horizon and are instrumental for the subsequent error analysis. Moreover, we dwell on extensions of Besov regularity to the fractional $p$-Laplacian, and review the regularity of fractional minimal graphs and stickiness. We discretize these problems using continuous piecewise linear finite elements and derive global and local error estimates for linear problems, thereby improving some existing error estimates for both quasi-uniform and graded meshes. We also present a BPX preconditioner which turns out to be robust with respect to both the fractional order and the number of levels. We conclude with the discretization of fractional quasi-linear problems and their error analysis. We illustrate the theory with several illuminating numerical experiments.