$S$-integral quadratic forms and homogeneous dynamics

Abstract

Let $S=\{\infty \}\cup S_f$ be a finite set of places of $\mathbb{Q}$. Using homogeneous dynamics, we establish two new quantitative and explicit results about integral quadratic forms in three or more variables: The first is a criterion of $S$-integral equivalence. The second determines a finite generating set of any $S$-integral orthogonal group. Both theorems--which extend results of H. Li and G. Margulis for $S=\{\infty \}$--are given by polynomial bounds on the size of the coefficients of the quadratic forms.

arXiv commentWe add in the introduction a discussion of the extension of the results to any number field. Some misprints corrected