The relational complexity of linear groups acting on subspaces

Abstract: The relational complexity of a subgroup $G$ of $\mathrm{Sym}(\Omega)$ is a measure of the way in which the orbits of $G$ on $\Omega^k$ for various $k$ determine the original action of $G$. Very few precise values of relational complexity are known. This paper determines the exact relational complexity of all groups lying between $\mathrm{PSL}{n}(\mathbb{F})$ and $\mathrm{PGL}{n}(\mathbb{F})$, for an arbitrary field $\mathbb{F}$, acting on the set of $1$-dimensional subspaces of $\mathbb{F}^n$. We also bound the relational complexity of all groups lying between $\mathrm{PSL}{n}(q)$ and $\mathrm{P}\Gamma\mathrm{L}{n}(q)$, and generalise these results to the action on $m$-spaces for $m \ge 1$.