In the rest of the problem, we denote by $\|\cdot\|$ the norm associated with the inner product $(A, B) \mapsto \operatorname{Tr}({}^t A \times B)$.
For $n \in \mathbb{N}^*$ and $A \in \mathcal{M}_d(\mathbb{R})$, compare $\left\|A^n\right\|$ and $\|A\|^n$.