On the convexity of numerical range over certain fields

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 $\sigma :L\to L$ fixing $K$. This sesquilinear form defines the numerical range $\mathrm{Num}(M)$ of any $n\times n$ matrix over $L$. In this paper we study the convexity of $\mathrm{Num}(M)$ (under certain assumptions on $K$ and/or $M$). Many of the results are for ordered fields.