A Jarn\'ik-type theorem for a problem of approximation by cubic polynomials

Abstract

For a given decreasing positive real function $\psi$, let $\mathcal{A}_n(\psi)$ be the set of real numbers for which there are infinitely many integer polynomials $P$ of degree up to $n$ such that $\left\lvert P(x) \right\rvert \leq \psi(\operatorname{H}(P))$. A theorem by Bernik states that $\mathcal{A}_n(\psi)$ has Hausdorff dimension $\frac{n+1}{w+1}$ in the special case $\psi(r) = r^{-w}$, while a theorem by Beresnevich, Dickinson and Velani implies that the Hausdorff measure $\operatorname{\mathcal{H}}^g(\mathcal{A}_n(\psi))=\infty$ when a certain series diverges. In this paper we prove the convergence counterpart of this result when $P$ has bounded discriminant, which leads to a complete solution when $n = 3$ and $\psi(r) = r^{-w}$.