An arithmetic topos for integer matrices

Abstract

We study the topos of sets equipped with an action of the monoid of regular $2 \times 2$ matrices over the integers. In particular, we show that the topos-theoretic points are given by the double quotient $\left. GL_2(\hat{\mathbb{Z}}) ~\middle\backslash~ M_2(\mathbb{A}_f)~\middle/~GL_2(\mathbb{Q})\right.$, so they classify the groups $\mathbb{Z}^2 \subseteq A \subseteq \mathbb{Q}^2$ up to isomorphism. We determine the topos automorphisms and then point out the relation with Conway's big picture and the work of Connes and Consani on the Arithmetic Site. As an application to number theory, we show that classifying extensions of $\mathbb{Q}$ by $\mathbb{Z}$ up to isomorphism relates to Goormaghtigh conjecture.