Coercive second-kind boundary integral equations for the Laplace Dirichlet problem on Lipschitz domains

We present new second-kind integral-equation formulations of the interior and exterior Dirichlet problems for Laplace’s equation. The operators in these formulations are both continuous and coercive on general Lipschitz domains in \(\mathbb {R}^d\) ${\mathbb{R}}^d$ , \(d\ge 2\) $d\ge 2$ , in the space \(L^2(\Gamma )\) $L^2(\mathrm{\Gamma })$ , where \(\Gamma \) $\mathrm{\Gamma }$ denotes the boundary of the domain. These properties of continuity and coercivity immediately imply that (1) the Galerkin method converges when applied to these formulations; and (2) the Galerkin matrices are well-conditioned as the discretisation is refined, without the need for operator preconditioning (and we prove a corresponding result about the convergence of GMRES). The main significance of these results is that it was recently proved (see Chandler-Wilde and Spence in Numer Math 150(2):299–371, 2022) that there exist 2- and 3-d Lipschitz domains and 3-d star-shaped Lipschitz polyhedra for which the operators in the standard second-kind integral-equation formulations for Laplace’s equation (involving the double-layer potential and its adjoint) cannot be written as the sum of a coercive operator and a compact operator in the space \(L^2(\Gamma )\) $L^2(\mathrm{\Gamma })$ . Therefore there exist 2- and 3-d Lipschitz domains and 3-d star-shaped Lipschitz polyhedra for which Galerkin methods in \({L^2(\Gamma )}\) $L^2(\mathrm{\Gamma })$ do not converge when applied to the standard second-kind formulations, but do converge for the new formulations.