Division Algebras With Infinite Genus

TL;DR

This paper explores the limitations of using maximal subfield spectra to classify division algebras, constructing examples with infinite genus and revealing complex relationships between algebra isomorphism classes and their subfield spectra.

Contribution

It introduces the concept of linking field extensions and constructs division algebras with infinite genus, showing that maximal subfield spectra do not always determine algebra isomorphism classes.

Findings

Existence of division algebras with infinite genus.

Construction of fields with infinitely many nonisomorphic quaternion algebras.

Existence of fields where all quaternion division algebras are weakly isomorphic.

Abstract

To what extent does the maximal subfield spectrum of a division algebra determine the isomorphism class of that algebra? It has been shown that over some fields a quaternion division algebra's isomorphism class is largely if not entirely determined by its maximal subfield spectrum. However in this paper, we show that there are fields for which the maximal subfield spectrum says little to nothing about a quaternion division algebra's isomorphism class. We give an explicit construction of a division algebra with infinite genus. Along the way we introduce the notion of a "linking field extension," which we hope will be of independent interest. We go on to show that there exists a field K for which (1) there are infinitely many nonisomorphic quaternion division algebras with center K, and (2) any two quaternion division algebra with center K are pairwise weakly isomorphic. In fact we show that there are infinitely many nonisomorphic fields satisfying these two conditions.