Let $u$ solve $\Delta u+k^2u=0$ in $K$, with $u=0$, $|\nabla u|=1$, on $\partial K$, where the latter is a compact Lipschitz domain in $\mathbb R^3$
Then $K$ is a ball.
Solution Description
Two counterexamples have been published online on arXiv. Probably would need verification.
ArticleIs an originA computer-assisted counterexample to the planar Pompeiu and Schiffer conjectures
If you plan to formulate more than one problem all sharing the same background (e.g. they are all from the same paper) then please choose "Group", otherwise select "Single" option.
No remarks yet