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 $$\mathbb {R}^d$$ R d , $$d\ge 2$$ d ≥ 2 , in the space $$L^2(\Gamma )$$ L 2 ( Γ ) , where $$\Gamma $$ Γ 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 ( Γ ) . 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 ( Γ ) do not converge when applied to the standard second-kind formulations, but do converge for the new formulations.