The Erdős-Rado Sunflower Problem for Vector Spaces

Abstract

The famous Erdős-Rado sunflower conjecture suggests that an $s$-sun\-flower-free family of $k$-element sets has size at most $(Cs{)}^k$ for some absolute constant $C$. In this note, we investigate the analog problem for $k$-spaces over the field with $q$ elements. For $s\ge k+1$, we show that the largest $s$-sunflower-free family $\mathcal{F}$ satisfies \[ 1 \leq |\mathcal{F}| / q^{(s-1) \binom{k+1}{2} - k} \leq (q/(q-1))^k. \] For $s\le k$, we show that \[ q^{-\binom{k+1}{2}} \leq |\mathcal{F}| / q^{(s-1) \binom{k+1}{2} - k} \leq (q/(q-1))^k. \] Our lower bounds rely on an iterative construction that uses lifted maximum rank-distance (MRD) codes.

9 pages; minor corrections due to referee reports