Let $M \in M _ { n } ( \mathbb { R } )$, all of whose coefficients are non-negative. Show that $M$ is a stochastic matrix if and only if
$$M \left( \begin{array} { c } 1 \\ \vdots \\ 1 \end{array} \right) = \left( \begin{array} { c } 1 \\ \vdots \\ 1 \end{array} \right) .$$