Peres--Schlag's nonempty-interior problem and a shifted-product variant for product sets

Abstract

We study finite-field analogues of the Peres--Schlag nonempty-interior problem for product sets. Given \(A\subseteq\mathbb F_p\), we ask when a suitable one-dimensional linear image of \(A^n\) is full; equivalently, when there exist coefficients \(t_1,\ldots,t_n\in\mathbb F_p\) such that \[ t_1A+\cdots+t_nA=\mathbb F_p. \] For \(n\ge3\), we prove that, for every \(η>0\), this holds whenever \[ |A|\gg_{n,η} p^{\frac{3}{2n-1}+η}. \] This improves the exponent predicted by the direct product-set analogue of the Peres--Schlag threshold, namely \(|A|\gg p^{2/n}\). We also prove a two-dimensional near-half-density result. Motivated by sum-product phenomena, we also introduce and study a product-type variant in which linear forms are replaced by shifted product maps. We prove finite-field covering results for shifted products \[ (t_1 + A)(t_2 + A)\cdots(t_n + A) \] at the same density scale as in the linear case. Finally, we prove a Euclidean shifted-product analogue: if \(A\subseteq\mathbb R\) is Borel and \(\dim_H A>2/n\), then some shifted product of \(n\) copies of \(A\) contains a nonempty open interval.