The spectrum of a class of graphs derived from Grassmann graphs

Let $n,k$ be positive integers such that $n\geq 3$, $k < \frac {n}{2}$. Let $q$ be a power of a prime $p$ and $\mathbb{F}_q$ be a finite field of order $q$. Let $V(q,n)$ be a vector space of dimension $n$ over $\mathbb{F}_q$. We define the graph $S(q,n,k)$ as a graph with the vertex set $V=V_k \cup V_{k+1}$, where $V_k$ and $V_{k+1}$ are the family of subspaces in $V(q,n)$ of dimension $k$ and $k+1$ respectively, in which two vertices $v$ and $w$ are adjacent whenever $v$ is a subspace of $w$ or $w$ is a subspace of $v$. It is clear that the graph $S(q,n,k)$ is a bipartite graph. In this paper, we study some properties of this graph. In particular, we determine the spectrum of the graph $S(q,n,k)$.