Nontrivial solutions to the relative overdetermined torsion problem in a cylinder

Abstract: Given a bounded regular domain $\omega \subset \mathbb{R}^{N-1}$ and the half-cylinder $\Sigma = \omega \times (0,+\infty)$, we consider the relative overdetermined torsion problem in $\Sigma$, i.e. [\left{ \begin{array}{ll} \Delta {u}+1=0 &\mbox{in $\Omega$},\newline \partial_\eta u = 0 &\mbox{on $\widetilde \Gamma_\Omega$},\newline u=0 &\mbox{on $\Gamma_\Omega$},\newline \partial_{\nu}u =c &\mbox{on $\Gamma_\Omega$}. \end{array} \right. ] where $\Omega \subset \Sigma$, $\Gamma_\Omega = \partial \Omega \cap \Sigma$, $\widetilde \Gamma_\Omega = \partial \Omega \setminus \Gamma_\Omega$, $\nu$ is the outer unit normal vector on $\Gamma_\Omega$ and $\eta$ is the outer unit normal vector on $\widetilde \Gamma_\Omega$. We build nontrivial solutions to this problem in domains $\Omega$ that are the hypograph of certain nonconstant functions $v : \overline{\omega} \to (0, + \infty)$. Such solutions can be reflected with respect to $\omega$, giving nontrivial solutions to the relative overdetermined torsion problem in a cylinder. The proof uses a local bifurcation argument which, quite remarkably, works for any generic base $\omega$.