Bounded Generation by semi-simple elements: quantitative results

TL;DR

This paper establishes that sets with purely exponential parametrizations, such as matrices boundedly generated by semi-simple elements, have at most logarithmic size distribution when ordered by height, revealing structural constraints on linear groups.

Contribution

It provides a quantitative bound on the distribution of points with exponential parametrizations and characterizes linear groups admitting such parametrizations as finitely generated with a torus Zariski closure.

Findings

Distribution of points is at most logarithmic in size

Linear groups with exponential parametrizations are finitely generated with a torus Zariski closure

Key inequality relates heights of minimal tuples to exponential parametrizations

Abstract

We prove that for a number field $F$, the distribution of the points of a set $\Sigma \subset \mathbb{A}_F^n$ with a purely exponential parametrization, for example a set of matrices boundedly generated by semi-simple (diagonalizable) elements, is of at most logarithmic size when ordered by height. As a consequence, one obtains that a linear group $\Gamma\subset \mathrm{GL}_n(K)$ over a field $K$ of characteristic zero admits a purely exponential parametrization if and only if it is finitely generated and the connected component of its Zariski closure is a torus. Our results are obtained via a key inequality about the heights of minimal $m$-tuples for purely exponential parametrizations. One main ingredient of our proof is Evertse's strengthening of the $S$-Unit Equation Theorem.