Convergence to the Mahler measure and the distribution of periodic points for algebraic Noetherian $\mathbb{Z}^d$-actions (1611.04664v2)

Abstract: For every $P \in \mathbb{Z}[x_1^{\pm 1}, \ldots, x_d^{\pm 1}] \setminus {0}$, and every $\varepsilon > 0$, we prove that there are a computable function $M = M(d,\varepsilon,\deg{P},h(P)) < \infty$ and a finite union $Z = Z(d,\varepsilon,\deg{P},h(P))$ of proper torsion cosets $\boldsymbol{\mu} T \subsetneq \mathbb{G}_m^d$ such that, for every $N \in \mathbb{N}$, $Z$ contains all but at most $M$ of the torsion points $\boldsymbol{\zeta} \in \mu_N^d$ satisfying $|P(\boldsymbol{\zeta})| < e^{-\varepsilon \phi(N)}$. This extends a well known structural theorem from torsion points lying exactly on a variety to torsion points lying very near to the subvariety. As a consequence, we prove that the averages of $\log{|P(\mathbf{x})|}$ over $\mu_N^d$ converge as $N \to \infty$ to the Mahler measure of $P$. By the work of B. Kitchens, D. Lind, K. Schmidt and T. Ward, the convergence consequence amounts to the following statement in dynamics: For every Noetherian $\mathbb{Z}^d$-action $T : \mathbb{Z}^d \to \mathrm{Aut}(X)$ by automorphisms of a compact abelian group $X$ having a finite topological entropy $h(T)$, the exponential growth rate of the number of connected components of the group $\mathrm{Per}_N(T)$ of $N \cdot \mathbb{Z}^d$-periodic points of $(X,T)$ exists as $N \to \infty$, and equals the topological entropy $h(T)$. Moreover, it follows that all weak-$*$ limit measures of the push-forwards of the Haar measures on $\mathrm{Per}_N(T)$, under any a sequence of positive integers $N$, are measures of maximum entropy $h(T)$. Our main arithmetic result extends to Diophantine approximation by points of sufficiently small canonical height. It is best possible in such a generality, where an exceptional set is an inevitable feature.