Equidistribution of primitive vectors, and the shortest solutions to their GCD equations

Abstract: We prove effective joint equidistribution of several natural parameters associated to primitive vectors in $\mathbb{Z}^{n}$, as the norm of these vectors tends to infinity. These parameters include the direction, the orthogonal lattice, and the length of the shortest solution to the associated $\gcd$ equation. We show that the first two parameters equidistribute w.r.t. the Haar measure on the corresponding spaces, which are the unit sphere and the space of unimodular rank $n-1$ lattices in $\mathbb{R}^{n}$ respectively. The main novelty is the equidistribution of the shortest solutions to the $\gcd$ equations: we show that, when normalized by the covering radius of the orthogonal lattice, the lengths of these solutions equidistribute in the interval $\left[0,1\right]$ w.r.t. a measure that is Lebesgue only when $n=2$, and non-Lebesgue otherwise. These equidistribution results are deduced from effectively counting lattice points in domains which are defined w.r.t. a generalization of the Iwasawa decomposition in simple algebraic Lie groups, where we apply a method due to A. Gorodnik and A. Nevo.