On the minimum size of linear sets

Abstract

Recently, a lower bound was established on the size of linear sets in projective spaces, that intersect a hyperplane in a canonical subgeometry. There are several constructions showing that this bound is tight. In this paper, we generalize this bound to linear sets meeting some subspace $\pi$ in a canonical subgeometry. We obtain a tight lower bound on the size of any ${\mathbb{F}}_q$-linear set spanning $\text{PG}(d,q^n)$ in case that $n\le q$ and $n$ is prime. We also give constructions of linear sets attaining equality in the former bound, both in the case that $\pi$ is a hyperplane, and in the case that $\pi$ is a lower dimensional subspace.

arXiv comment24 pages; the updated version contains a consequences of the recent paper by Csajbók, Marino and Pepe (arXiv:2306.07488), providing a tight lower bound in some cases