Let $A \in \mathcal { S } _ { N } ( \mathbb { R } )$ and let $P \in \mathbb { R } [ X ]$ be a polynomial. Show that $P ( A ) \in \mathcal { S } _ { N } ( \mathbb { R } )$ and specify the eigenvalues and eigenvectors of $P ( A )$ in terms of those of $A$.