Boundary elliptic regularity for the shifted Dirichlet Laplacian
= Boundary elliptic regularity for the shifted Dirichlet Laplacian
On a smooth bounded domain, a solution of
$$
(\Delta-c)u=f,\qquad u|_{\partial\Omega}=0,
$$
for an invertible shift satisfies
$$
\|u\|_{H^{k+2}}\leq C_k\|f\|_{H^k}.
$$
Interior regularity and boundary flattening give the derivative gain, while invertibility controls the lower-order norm.