Economical lattice coverings by determined segments

Abstract

For fixed $d\geq 2$, let $τ_d(n)$ be the minimum size of a set $S\subseteq\{0,\ldots,n\}^d$ such that the affine lines determined by pairs of distinct points of $S$ cover the grid. Let $σ_d(n)$ be the analogous minimum when every grid point must lie on the closed segment joining two distinct points of $S$. A celebrated result of Alon [GAFA, 1991] proved that $τ_d(n)$ is of order between $Ω_d(n^{α_d})$ and $O_d(n^{α_d}\log n)$, where $α_d=\frac{d(d-1)}{2d-1}$, and asked whether the logarithm term is necessary. We prove that $$c_d n^{α_d}\leqτ_d(n)\leqσ_d(n)\leq C_d n^{α_d}$$ for every fixed $d\geq 2$, thereby resolving Alon's problem in a stronger form.