Dimension estimates for badly approximable affine forms (2111.15410v2)

Abstract: For given $\epsilon>0$ and $b\in\mathbb{R}^m$, we say that a real $m\times n$ matrix $A$ is $\epsilon$-badly approximable for the target $b$ if $$\liminf_{q\in\mathbb{Z}^n, |q|\to\infty} |q|^n \langle Aq-b \rangle^m \geq \epsilon,$$ where $\langle \cdot \rangle$ denotes the distance from the nearest integral point. In this article, we obtain upper bounds for the Hausdorff dimensions of the set of $\epsilon$-badly approximable matrices for fixed target $b$ and the set of $\epsilon$-badly approximable targets for fixed matrix $A$. Moreover, we give an equivalent Diophantine condition of $A$ for which the set of $\epsilon$-badly approximable targets for fixed $A$ has full Hausdorff dimension for some $\epsilon>0$. The upper bounds are established by effectivizing entropy rigidity in homogeneous dynamics, which is of independent interest. For the $A$-fixed case, our method also works for the weighted setting where the supremum norms are replaced by certain weighted quasinorms.