Local-global principles for multinorm tori over semi-global fields

Abstract

Let $K$ be a complete discretely valued field with the residue field $κ$. Assume that cohomological dimension of $κ$ is less than or equal to $1$ (for example, $κ$ is an algebraically closed field or a finite field). Let $F$ be the function field of a curve over $K$. Let $n$ be a squarefree positive integer not divisible by char$(κ)$. Then for any two degree $n$ abelian extensions, we prove that the local-global principle holds for the associated multinorm torus with respect to discrete valuations. Let $\mathscr{X}$ be a regular proper model of $F$ such that the reduced special fibre $X$ is a union of regular curves with normal crossings. Suppose that $κ$ is algebraically closed with $char(κ)\neq 2$. If the graph associated to $\mathscr{X}$ is a tree (e.g. $F = K(t)$) then we show that the same local-global principle holds for the multinorm torus associated to finitely many abelian extensions where one of the extensions is quadratic and others are of degree not divisible by $4$.