Cayley algebras give rise to $q$-Fano planes over certain infinite fields and $q$-covering designs over others

Abstract

Let $F$ be a field. A $2$-$(7, 3, 1)_F$-subspace design, or $q$-Fano plane, over $F$, is a $7$-dimensional vector space $V$ over $F$ together with a collection $\mathfrak{B}$ of three-dimensional subspaces of $V$ such that every two-dimensional subspace of $V$ is contained in exactly one element $B$ of $\mathfrak{B}$. The question of existence of $q$-Fano planes over any field has been open since the 1970s and has attracted considerable attention in the special case that $F$ is finite. Here we show the existence of $2$-$(7, 3, 1)_F$-subspace designs over certain infinite fields $F$, including (among others) $\mathbb{Q}, \mathbb{R}$ and $\mathbb{F}_q(x, y, z)$ for $q$ odd. The space $V$ is the 7-dimensional space of imaginary elements in a Cayley division algebra $O$ over $F$ and $\mathfrak{B}$ consists of the intersections with $V$ of all 4-dimensional subalgebras of $O$. We will present the required background on Cayley algebras in a self-contained fashion. Next we study what happens if we apply the same procedure to split (rather than division) Cayley algebras. By identifying all four-dimensional subalgebras of these, we show that in that case our construction still yields an inclusion minimal $(7, 3, 2)$ $q$-covering design. That is: every two-dimensional subspace of $V$ is contained in at least one element of the resulting set $\mathfrak{B}$ of three-dimensional subspaces of $V$ and no proper subset of $\mathfrak{B}$ has this property. However none of these $q$-covering designs are $q$-Fano planes. In the case that $F$ is finite we compute the number of elements of $\mathfrak{B}$. We also give a purely combinatorial construction of our $q$-Fano planes and $q$-covering designs for an abstract 7-dimensional $F$-vector space $V$ by identifying the collection $\mathfrak{B}$ as a subvariety of the Grassmanian $Gr_3(V)$ defined entirely in terms of the classical Fano plane.