Balanced intersection size distributions in finite geometries and vector spaces

Abstract

We study intersection size distributions arising in finite geometries and vector spaces. For a projective plane $\Pi_q$ of order $q$ with line set $\mathcal L$, we show that \[ \min_{S\subseteq\Pi_q}\max_k |\{\ell\in\mathcal L:|S\cap\ell|=k\}|=\Theta(q^{3/2}). \] The arguments behind these bounds admit a more general formulation. We prove a similar lower bound for balanced incomplete block designs, together with probabilistic upper bounds for uniform block systems: one for fixed block size under a weak dependency condition and another for growing block size under bounded pairwise intersections. This yields corresponding results for subspaces of vector spaces over finite fields, equivalently, for affine subspaces of an affine space. We also study explicit constructions in affine planes of prime order and relate their intersection size distributions to character sum estimates; an elliptic curve construction gives a bound within a polylogarithmic factor of the optimal order. Finally, we relate balanced intersection size distributions to legitimate colorings and prove that every n-uniform linear hypergraph with $n$ edges admits a legitimate 2-coloring.