The Dirichlet-to-Neumann operator via hidden compactness

We show that to each symmetric elliptic operator of the form $$\mathcal{A} = - \sum \partial_k \, a_{kl} \, \partial_l + c$$ on a bounded Lipschitz domain $\Omega \subset \mathbb{R}^d$ one can associate a self-adjoint Dirichlet-to-Neumann operator on $L_2(\partial \Omega)$, which may be multi-valued if 0 is in the Dirichlet spectrum of $\mathcal{A}$. To overcome the lack of coerciveness in this case, we employ a new version of the Lax--Milgram lemma based on an indirect ellipticity property that we call hidden compactness. We then establish uniform resolvent convergence of a sequence of Dirichlet-to-Neumann operators whenever their coefficients converge uniformly and the second-order limit operator in $L_2(\Omega)$ has the unique continuation property. We also consider semigroup convergence.