Companion Weakly Periodic Matrices over Finite and Countable Fields

We explore the situation where all companion $n\times n$ matrices over a field $F$ are weakly periodic of index of nilpotence $2$ and prove that this can be happen uniquely when $F$ is a countable field of positive characteristic, which is an algebraic extension of its minimal simple (finite) subfield, with all subfields of order greater than $n$. In particular, in the commuting case, we show even that $F$ is a finite field of order greater than $n$. Our obtained results somewhat generalize those obtained by Breaz-Modoi in Lin. Algebra & Appl. (2016).