Graph decompositions in projective geometries

Abstract

Let PG$(\mathbb{F}_q^v)$ be the $(v-1)$-dimensional projective space over $\mathbb{F}_q$ and let $Γ$ be a simple graph of order ${q^k-1\over q-1}$ for some $k$. A 2$-(v,Γ,λ)$ design over $\mathbb{F}_q$ is a collection $\cal B$ of graphs (\textit{blocks}) isomorphic to $Γ$ with the following properties: the vertex set of every block is a subspace of PG$(\mathbb{F}_q^v)$; every two distinct points of PG$(\mathbb{F}_q^v)$ are adjacent in exactly $λ$ blocks. This new definition covers, in particular, the well known concept of a 2$-(v,k,λ)$ design over $\mathbb{F}_q$ corresponding to the case that $Γ$ is complete. In this work of a foundational nature we illustrate how difference methods allow us to get concrete non-trivial examples of $Γ$-decompositions over $\mathbb{F}_2$ or $\mathbb{F}_3$ for which $Γ$ is a cycle, a path, a prism, a generalized Petersen graph, or a Moebius ladder. In particular, we will discuss in detail the special and very hard case that $Γ$ is complete and $λ=1$, i.e., the Steiner 2-designs over a finite field. Also, we briefly touch the new topic of near resolvable 2-$(v,2,1)$ designs over $\mathbb{F}_q$. This study has led us to some (probably new) collateral problems concerning difference sets. Supported by multiple examples, we conjecture the existence of infinite families of $Γ$-decompositions over a finite field that can be obtained by suitably labeling the vertices of $Γ$ with the elements of a Singer difference set.