Lemma 1.11. - ${\phi }_1$ is an eigenfunction of ${\lambda }_1$. Then: ${\lambda }_2(M)=\underset{\begin{array}{c}f\perp {\phi }_1\\ f\ne 0\end{array}}{\min }{q}_M(f)$, and it is attained only on eigenfunctions of ${\lambda }_2(M)$.