Evasive subspaces

Abstract

Let $V$ denote an $r$-dimensional vector space over ${\mathbb{F}}_{q^n}$, the finite field of $q^n$ elements. Then $V$ is also an $rn$-dimension vector space over ${\mathbb{F}}_q$. An ${\mathbb{F}}_q$-subspace $U$ of $V$ is $(h,k{)}_q$-evasive if it meets the $h$-dimensional ${\mathbb{F}}_{q^n}$-subspaces of $V$ in ${\mathbb{F}}_q$-subspaces of dimension at most $k$. The $(1,1{)}_q$-evasive subspaces are known as scattered and they have been intensively studied in finite geometry, their maximum size has been proved to be $\lfloor rn/2\rfloor$ when $rn$ is even or $n=3$. We investigate the maximum size of $(h,k{)}_q$-evasive subspaces, study two duality relations among them and provide various constructions. In particular, we present the first examples, for infinitely many values of $q$, of maximum scattered subspaces when $r=3$ and $n=5$. We obtain these examples in characteristics $2$, $3$ and $5$.

Revised version according to the referees' suggestions. We also added some connections with q-systems. Theorem 4.3 and Remark 5.2 are new. Accepted by the Journal of Combinatorial Designs