# On quasi-reversibility solutions to the Cauchy problem for the Laplace equation: regularity and error estimates

## TL;DR

This paper investigates regularity and error estimates for quasi-reversibility solutions to the ill-posed Cauchy problem for the Laplace equation, providing uniform results in regularized problems in smooth and polygonal domains.

## Contribution

It develops uniform regularity results for regularized Laplace problems in smooth and polygonal domains, including corners, using Kondratiev techniques, with detailed dependence on the regularization parameter.

## Key findings

- Established regularity results in smooth domains.
- Extended analysis to 2D polygonal geometries with corners.
- Provided detailed ε-dependent estimates for regularized solutions.

## Abstract

We are interested in the classical ill-posed Cauchy problem for the Laplace equation. One method to approximate the solution associated with compatible data consists in considering a family of regularized well-posed problems depending on a small parameter $\varepsilon>0$. In this context, in order to prove convergence of finite elements methods, it is necessary to get regularity results of the solutions to these regularized problems which hold uniformly in $\varepsilon$. In the present work, we obtain these results in smooth domains and in 2D polygonal geometries. In presence of corners, due the particular structure of the regularized problems, classical techniques \`a la Grisvard do not work and instead, we apply the Kondratiev approach. We describe the procedure in detail to keep track of the dependence in $\varepsilon$ in all the estimates. The main originality of this study lies in the fact that the limit problem is ill-posed in any framework.