Differences of squares of upper-triangular $2\times 2$ integer matrices

TL;DR

This paper characterizes upper-triangular 2x2 integer matrices that can be expressed as the difference of squares of two upper-triangular matrices, providing criteria based on number theory and divisibility conditions.

Contribution

It offers a complete characterization and classification of such matrices using difference-of-squares representations and congruence conditions.

Findings

Matrices with p and q as differences of squares are characterized.

Divisibility conditions on r determine representability.

Complete classification based on congruence conditions.

Abstract

We consider the problem of characterizing upper-triangular matrices $M=\begin{pmatrix}p&r\\0&q\end{pmatrix}\in M_2(\mathbb Z)$ which can be represented in the form $A^2-B^2$ with upper-triangular integer matrices $A$ and $B$ and give a complete criterion in terms of representations of $p$ and $q$ as differences of two squares and an additional divisibility condition on $r$. Also, we give a complete classification of representable matrices in terms of congruence conditions on $p$, $q$, and $r$.