The Fra\"iss\'e limit of matrix algebras with the rank metric

Abstract

We realize the $\mathbb{F}_q$-algebra $M(\mathbb{F}_q)$ studied by von Neumann and Halperin as the Fra\"iss\'e limit of the class of finite-dimensional matrix algebras over a finite field $\mathbb{F}_q$ equipped with the rank metric. We then provide a new Fra\"iss\'e-theoretic proof of uniqueness of such an object. Using the results of Carderi and Thom, we show that the automorphism group of $\mathrm{Aut}(\mathbb{F}_q )$ is extremely amenable. We deduce a Ramsey-theoretic property for the class of algebras $M(\mathbb{F}_q)$, and provide an explicit bound for the quantities involved.