So, I have been trying to solve the following problem: find the ring of integers in $\mathbb{Q}(\sqrt{3}+\sqrt{7}).$
Of course, the answer for $\mathbb{Q}(\sqrt{3})$ is clear as is for $\mathbb{Q}(\sqrt{7}),$ you just look for the residue modulo $4$ and you get the answer.
However, then I got stuck because I don't know how to derive an analogous criterion for quadratic extensions of $\mathbb{Q}(\sqrt{d}).$ Can anyone suggest an approach? Thank you!