$L^\infty$ norm error estimates for HDG methods applied to the Poisson equation with an application to the Dirichlet boundary control problem

TL;DR

This paper establishes the first $L^ Infty$ norm error estimates for the HDG method applied to Poisson's problem, demonstrating optimal convergence and validating results through numerical experiments.

Contribution

It provides the first proof of $L^ Infty$ norm error estimates for HDG methods, extending known results from conforming and mixed finite element methods.

Findings

Proves quasi-optimal $L^ Infty$ error estimates for HDG methods.

Shows optimal convergence rates for boundary flux estimates.

Numerical experiments confirm theoretical convergence rates.

Abstract

We prove quasi-optimal $L^\infty$ norm error estimates (up to logarithmic factors) for the solution of Poisson's problem by the standard Hybridizable Discontinuous Galerkin (HDG) method. Although such estimates are available for conforming and mixed finite element methods, this is the first proof for HDG. The method of proof is motivated by known $L^\infty$ norm estimates for mixed finite elements. We show two applications: the first is to prove optimal convergence rates for boundary flux estimates, and the second is to prove that numerically observed convergence rates for the solution of a Dirichlet boundary control problem are to be expected theoretically. Numerical examples show that the predicted rates are seen in practice.