Bipartite Q-polynomial distance-regular graphs and uniform posets

Abstract

Let $\G$ denote a bipartite distance-regular graph with vertex set $X$ and diameter $D\ge 3$. Fix $x\in X$ and let $L$ (resp. $R$) denote the corresponding lowering (resp. raising) matrix. We show that each $Q$-polynomial structure for $\G$ yields a certain linear dependency among $R{L}^2$, $LRL$, $L^{2}R$, $L$. Define a partial order $\le$ on $X$ as follows. For $y,z\in X$ let $y\le z$ whenever $\partial (x,y)+\partial (y,z)=\partial (x,z)$, where $\partial$ denotes path-length distance. We determine whether the above linear dependency gives this poset a uniform or strongly uniform structure. We show that except for one special case a uniform structure is attained, and except for three special cases a strongly uniform structure is attained.