A characterization of the graphs of bilinear $(d\times d)$-forms over $\mathbb{F}_2$

Abstract

The bilinear forms graph denoted here by $Bil_q(e\times d)$ is a graph defined on the set of $(e\times d)$-matrices ($e\geq d$) over $\mathbb{F}_q$ with two matrices being adjacent if and only if the rank of their difference equals $1$. In 1999, K. Metsch showed that the bilinear forms graph $Bil_q(e\times d)$ is characterized by its intersection array if one of the following holds: (-) $q=2$ and $e\geq d+4$, (-) $q\geq 3$ and $e\geq d+3$. Thus, the following cases have been left unsettled: (-) $q=2$ and $e\in \{d,d+1,d+2,d+3\}$, (-) $q\geq 3$ and $e\in \{d,d+1,d+2\}$. In this work, we show that the graph of bilinear $(d\times d)$-forms over the binary field, where $d\geq 3$, is characterized by its intersection array. In doing so, we also classify locally grid graphs whose $\mu$-graphs are hexagons and the intersection numbers $b_i,c_i$ are well-defined for all $i=0,1,2$.