Exact Generic Exponents of Approximation for the Action of the Integral Special Linear Group on the Torus
Given an integer $n\ge 2$ and points $\mathbf{x}, \mathbf{y}$ lying in the $n$--dimensional torus $\mathbb{T}^n$ equipped with its usual metric denoted by $\left\langle\;\cdot\;\right\rangle$, let \[ τ_n(\mathbf x,\mathbf y) = \sup\left\{ τ\ge0: \begin{array}{l} \text{there exist infinitely many } H\ge1\text{ for which}\\[1mm] \|A\|\le H \text{ and } \langle A\mathbf x-\mathbf y\rangle\le H^{-τ} \text{ for some } A\in\mathrm{SL}_n(\mathbb Z) \end{array} \right\} \] be the Diophantine exponent of asymptotic approximation of $\mathbf{y}$ by the orbit of $\mathbf{x}$ under the action of the integral special linear group $\mathrm{SL}_n(\mathbb Z)$. Let also \[ \widehatτ_n\!\left(\mathbf{x}, \mathbf{y}\right) = \sup\left\{ τ\ge0: \begin{array}{l} \text{for all but finitely many } H\ge1, \text{ there is } A\in\mathrm{SL}_n(\mathbb Z)\\[1mm] \text{such that } \|A\|\le H \text{ and } \langle A\mathbf x-\mathbf y\rangle\le H^{-τ} \end{array} \right\} \] denote the corresponding exponent of uniform approximation. The main result of this paper proves that for almost all $\mathbf{x}, \mathbf{y}\in\mathbb{T}^n$ (with respect to the induced Lebesgue measure), $$\widehatτ_n\!\left(\mathbf{x}, \mathbf{y}\right)\;=\; τ_n\!\left(\mathbf{x}, \mathbf{y}\right)\;=\; n-1.$$ This is the first result to establish the exact generic approximation exponents for an action of a lattice subgroup of $\mathrm{SL}_n(\mathbb R)$ induced by the standard Euclidean action.