$\mathbf{X}[k+1]=A\mathbf {X}[k]+B\mathbf{u}[k]$
$ \left(\, \sum_{k=1}^n a_k b_k \right)^{\!\!2} \le $ $\left(\, \sum_{k=1}^n a_k^2 \right) \left(\, \sum_{k=1}^n b_k^2 \right)$
\[ \left(\, \sum_{k=1}^n a_k b_k \right)^{\!\!2} \le \]