# How to explain this afirmation?

Consider $\Omega=\mathbf{R}_+^n$ the half-space and $u$ a function that satisfies: $$0<u<\sup u=M<\infty,$$ $$\Delta u+u-1=0, \ \ in \ \ \Omega,$$ $$u=0, \ \ on \ \ \partial\Omega.$$ Then $M>2$.

-
 Where did you get this problem? – Tomás Oct 22 '12 at 21:29