The Matrix Pythagorean Equation over ${\mathrm{GL}}_2(\mathbb{Z})$

Abstract

In this paper, we study the ordered solutions of the matrix Pythagorean equation $X^2+Y^2=Z^2$ over ${\mathrm{GL}}_2(\mathbb{Z})$ . Exploiting the independent sign symmetry of the equation, we first reduce the full solution set to the trace-nonnegative subset $\mathcal{M}$, consisting of those solutions for which all three traces are nonnegative. We then determine a canonical decomposition of $\mathcal{M}$ into orbits under simultaneous integral conjugacy.