On the numerical range of square matrices with coefficients in a degree 2 Galois field extension

Abstract

Let $L$ be a degree $2$ Galois extension of the field $K$ and $M$ an $n\times n$ matrix with coefficients in $L$. Let $⟨,⟩:L^n\times L^n\to L$ be the sesquilinear form associated to the involution $L\to L$ fixing $K$. We use $⟨,⟩$ to define the numerical range $\mathrm{Num}(M)$ of $M$ (a subset of $L$), extending the classical case $K=\mathbb{R}$, $L=\mathbb{C}$ and the case of a finite field introduced by Coons, Jenkins, Knowles, Luke and Rault. There are big differences with respect to both cases for number fields and for all fields in which the image of the norm map $L\to K$ is not closed by addition, e.g., $c\in L$ may be an eigenvalue of $M$, but $c\notin \mathrm{Num}(M)$. We compute $\mathrm{Num}(M)$ in some case, mostly with $n=2$.