Pairs of commuting integer matrices

Abstract

We prove upper and lower bounds on the number of pairs of commuting $n\times n$ matrices with integer entries in $[-T,T]$, as $T\to \infty$. Our work uses Fourier analysis and leads us to an analysis of exponential sums involving matrices over finite fields. These are bounded by combining a stratification result of Fouvry and Katz with a new result about the flatness of the commutator Lie bracket.

15 pages. More background information is included on flatness; a new theorem on point counting via exponential sums is extracted in Theorem 4.1