Nontrivial $t$-designs in polar spaces exist for all $t$

Abstract

A finite classical polar space of rank $n$ consists of the totally isotropic subspaces of a finite vector space over ${\mathbb{F}}_q$ equipped with a nondegenerate form such that $n$ is the maximal dimension of such a subspace. A $t$-$(n,k,\lambda )$ design in a finite classical polar space of rank $n$ is a collection $Y$ of totally isotropic $k$-spaces such that each totally isotropic $t$-space is contained in exactly $\lambda$ members of $Y$. Nontrivial examples are currently only known for $t\le 2$. We show that $t$-$(n,k,\lambda )$ designs in polar spaces exist for all $t$ and $q$ provided that $k>\frac{21}{2}t$ and $n$ is sufficiently large enough. The proof is based on a probabilistic method by Kuperberg, Lovett, and Peled, and it is thus nonconstructive.

11 pages; minor changes taking into account referee comments