The first argument is based on p-adic analysis and may be viewed as an extension of two classical strategies from a linear to a non-linear context. The first strategy appeared in the proof of the theorem of Skolem, Mahler, and Lech, which says that the zeros of a linear recurrence sequence occur along a finite union of arithmetic progressions. This method plays now a central role in arithmetic dynamics (see [6] , [7], [55]). The second strategy has been developed by Bass, Milnor, and Serre when they obtained rigidity results for finite-dimensional linear representations of SL n (Z) as a corollary of the congruence subgroup property (see [2], [62]). Here, we combine these strategies for non-linear actions of finitely generated groups of birational transformations.Our second argument makes use of isoperimetric inequalities from geometric group theory and of the Lang-Weil estimates from diophantine geometry. We now list the main results that follow from the combination of those arguments.
Actions of SL n (Z)Consider the group SL n (Z) of n×n matrices with integer entries and determinant 1. Let Γ be a finite-index subgroup of SL n (Z); it acts by linear projective transformations on the projective space P n−1 C , and the kernel of the homomorphism Γ!PGL n (C) contains at most two elements. The following result shows that Γ does not act faithfully on any smaller variety.