# norm

Let $K/F$ be a Galois extension, and let $x\in K$. The norm $\operatorname{N}_{F}^{K}(x)$ of $x$ is defined to be the product of all the elements of the orbit of $x$ under the group action of the Galois group $\operatorname{Gal}(K/F)$ on $K$; taken with multiplicities if $K/F$ is a finite extension.

In the case where $K/F$ is a finite extension, the norm of $x$ can be defined to be the determinant of the linear transformation $[x]:K\to K$ given by $[x](k):=xk$, where $K$ is regarded as a vector space over $F$. This definition does not require that $K/F$ be Galois, or even that $K$ be a field—for instance, it remains valid when $K$ is a division ring (although $F$ does have to be a field, in order for determinant to be defined). Of course, for finite Galois extensions $K/F$, this definition agrees with the previous one, and moreover the formula

 $\operatorname{N}_{F}^{K}(x):=\prod_{\sigma\in\operatorname{Gal}(K/F)}\sigma(x)$

holds.

The norm of $x$ is always an element of $F$, since any element of $\operatorname{Gal}(K/F)$ permutes the orbit of $x$ and thus fixes $\operatorname{N}_{F}^{K}(x)$.

Title norm Norm 2013-03-22 12:18:02 2013-03-22 12:18:02 djao (24) djao (24) 5 djao (24) Definition msc 12F05