The genus of division algebras over discrete valued fields

Abstract

Given a field with a set of discrete valuations $V$, we show how the genus of a division algebra over the field is related to the genus of the residue algebras at various valuations in $V$ and the ramification data. When the division algebra is a quaternion, we show the triviality of genus over many fields which include higher local fields, function fields of curves over higher local fields and function fields of curves over real closed fields. We also consider function fields of curves over global fields with a rational point and show how the genus problem is related to the $2$-torsion of the Tate-Shafarevich group of its Jacobian. As a special case, we show how the methods developed yield better bounds on the size of the genus over function fields of elliptic curves and demonstrate how they can be computed directly using arithmetic data of the elliptic curve with a number of examples.