Local Fields
《Local Fields》 · 第 23 页 · Chapter I，Section 7「Galois Extensions」
If $\mathfrak P$ is unramified over $\mathfrak p$ and $q$ is the number of elements of the residue field of $K$, there is a unique element $s_{\mathfrak P}$ of the decomposition group such that $s_{\mathfrak P}(b)\equiv b^q\pmod{\mathfrak P}$ for every integral $b$. It is called the Frobenius substitution belonging to $\mathfrak P$.