Ramsey numbers and extremal structures in polar spaces

John Bamberg, Anurag Bishnoi, Ferdinand Ihringer

Published 2024-06-05Version 1

We use $p$-rank bounds on partial ovoids and the classical bounds on Ramsey numbers to obtain various upper bounds on partial $m$-ovoids in finite polar spaces. These bounds imply non-existence of $m$-ovoids for various new families of polar spaces. We give a probabilistic construction of large partial $m$-ovoids when $m$ grows linearly with the rank of the polar space. In the special case of the symplectic spaces over the binary field, we show an equivalence between partial $m$-ovoids and a generalisation of the Oddtown theorem from extremal set theory that has been studied under the name of nearly $m$-orthogonal sets over finite fields. We give new constructions for partial $m$-ovoids in these spaces and thus $m$-nearly orthogonal sets, for small values of $m$. These constructions use triangle-free graphs whose complements have low $\mathbb{F}_2$-rank and we give an asymptotic improvement over the state of the art. We also prove new lower bounds in the recently introduced rank-Ramsey problem for triangles vs cliques