Let $A$ be a constant $n \times n$ matrix. Let $\mathbf{u}(t)$ be a solution to the system $\frac{d \mathbf{u}}{d t}=A \mathbf{u}$.
(a) Show that its derivatives $\frac{d^k \mathbf{u}}{d t^k}$ for $k=1,2, \ldots$, are also solutions,
(b) Show that $\frac{d^k \mathbf{u}}{d t^k}=A^k \mathbf{u}$.