Eigenpolynomials Associated with Subspaces in \(d\)-Bounded

Abstract

Let \(\Gamma\) denote a \(d\)-bounded distance-regular graph with diameter \(d \geq 2\). A regular strongly closed subgraph of \(\Gamma\) is said to be a subspace of \(\Gamma\). Define the empty set \(\emptyset\) to be the subspace with diameter \(-1\) in \(\Gamma\). For \(0 \leq i \leq d-1\), let \(\mathcal{L}(\leq i)\) (resp. \(\mathcal{L}(\geq i)\)) denote the set of all subspaces in \(\Gamma\) with diameters \(< i\) (resp. \(\geq i\)) including \(\Gamma\) and \(\emptyset\). If we define the partial order on \(\mathcal{L}(\leq i)\) (resp. \(\mathcal{L}(\geq i)\)) by reverse inclusion (resp. ordinary inclusion), then \(\mathcal{L}(\leq i)\) (resp. \(\mathcal{L}(\geq i)\)) is a poset, denoted by \(\mathcal{L}_R(\leq i)\) (resp. \(\mathcal{L}_o(\geq i)\)). In the present paper, we give the eigenpolynomials of \(\mathcal{L}_R(\leq i)\) and \(\mathcal{L}_o(\geq i)\).