Archimedean classes of matrices over ordered fields

Abstract

Let $(F,\le)$ be an ordered field and let $A,B$ be square matrices over $F$ of the same size. We say that $A$ and $B$ belong to the same archimedean class if there exists an integer $r$ such that the matrices $r A^T A-B^T B$ and $r B^T B-A^T A$ are positive semidefinite with respect to $\le$. We show that this is true if and only if $A=CB$ for some invertible matrix $C$ such that all entries of $C$ and $C^{-1}$ are bounded by some integer. We also show that every archimedean class contains a row echelon form and that its shape and archimedean classes (in $F$) of its pivots are uniquely determined. For matrices over fields of formal Laurent series we construct a canonical representative in each archimedean class. The set of all archimedean classes is shown to have a natural lattice structure while the semigroup structure does not come from matrix multiplication. Our motivation comes from noncommutative real algebraic geometry and noncommutative valuation theory.