Some orbits of a two-vertex stabilizer in a Grassmann graph

Let ${\mathbb{F}}_q$ denote a finite field with $q$ elements. Let $n,k$ denote integers with $n>2k\ge 6$. Let $V$ denote a vector space over ${\mathbb{F}}_q$ that has dimension $n$. The vertex set of the Grassmann graph $J_q(n,k)$ consists of the $k$-dimensional subspaces of $V$. Two vertices of $J_q(n,k)$ are adjacent whenever their intersection has dimension $k-1$. Let $\mathrm{\partial }$ denote the path-length distance function of $J_q(n,k)$. Pick vertices $x,y$ of $J_q(n,k)$ such that $1<\mathrm{\partial }(x,y)<k$. Let $\text{Stab}(x,y)$ denote the subgroup of $GL(V)$ that stabilizes both $x$ and $y$. In this paper, we investigate the orbits of $\text{Stab}(x,y)$ acting on the local graph $\mathrm{\Gamma }(x)$. We show that there are five orbits. By construction, these five orbits give an equitable partition of $\mathrm{\Gamma }(x)$; we find the corresponding structure constants. In order to describe the five orbits more deeply, we bring in a Euclidean representation of $J_q(n,k)$ associated with the second largest eigenvalue of $J_q(n,k)$. By construction, for each orbit its characteristic vector is represented by a vector in the associated Euclidean space. We compute many inner products and linear dependencies involving the five representing vectors.