Let $K=\mathbb{Q(\sqrt{d})}$ and  $\mathbb{a}$ be a nonzero ideal in the ring of integers $O_K$ . Why does the equation hold : $\mathbb{a}\mathbb{\overline a}=N(\mathbb{a}O_K)$ ? One can prove this by taking a prime ideal $\mathbb{a}=\mathbb{p}$  but this does not bring me further .