The ideal $\mathfrak p \cap\mathcal O_K$ is a nonzero prime ideal of $\Z$ (a prime of $\Q$), which is necessarily a principal ideal $(p)$ for some prime number $p$. The prime $\mathfrak p$ is then said to be a prime above $p$.
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- Last edited by John Cremona on 2019-10-29 15:26:52