Eigenfunctions for rectangles with Neumann boundary conditions

Abstract

Consider the eigenfunctions $u$ for a ree rectangular membrane wo that $-\Delta u=\lambda u$ on $\mathcal R(c,d)=(0,d)\times(0,d)$. In this note we show that if if $u>0$ on $\partial \mathcal R(c,d)) then $u\equiv C$ for some positive constant.