On matrices commuting with their Frobenius

Abstract

The Frobenius of a matrix $M$ with coefficients in ${\bar{\mathbb{F}}}_p$ is the matrix $\ddot{I}ƒ(M)$ obtained by raising each coefficient to the $p$-th power. We consider the question of counting matrices with coefficients in ${\mathbb{F}}_q$ which commute with their Frobenius, asymptotically when $q$ is a large power of $p$. We give answers for matrices of size $2$, for diagonalizable matrices, and for matrices whose eigenspaces are defined over ${\mathbb{F}}_p$. Moreover, we explain what is needed to solve the case of general matrices. We also solve (for both diagonalizable and general matrices) the corresponding problem when one counts matrices $M$ commuting with all the matrices $\ddot{I}ƒ(M)$, $\ddot{I}{ƒ}^2(M)$, $\dots$ in their Frobenius orbit.

minor changes, reordered sections; to appear in J. Algebra