Eigenbasis for a weighted adjacency matrix associated with the projective geometry $B_q(n)$

TL;DR

This paper provides an explicit eigenbasis for a weighted adjacency matrix related to projective geometry, offers new proofs for its eigenvalues, and demonstrates the $Q$-polynomial property through eigenvector analysis.

Contribution

It explicitly constructs an eigenbasis, evaluates eigenvector products to re-derive eigenvalues, and proves the $Q$-polynomial property via dual adjacency matrix analysis.

Findings

Explicit eigenbasis for the weighted adjacency matrix

New proof of eigenvalues and multiplicities

Demonstration of the $Q$-polynomial property

Abstract

In a recent article "Projective geometries, $Q$-polynomial structures, and quantum groups" Terwilliger (arXiv:2407.14964) defined a certain weighted adjacency matrix, depending on a free (positive real) parameter, associated with the projective geometry, and showed (among many other results) that it is diagonalizable, with the eigenvalues and their multiplicities explicitly written down, and that it satisfies the $Q$-polynomial property (with respect to the zero subspace). In this note we (i) Write down an explicit eigenbasis for this matrix. (ii) Evaluate the adjacency matrix-eigenvector products, yielding a new proof for the eigenvalues and their multiplicities. (iii) Evaluate the dual adjacency matrix-eigenvector products and directly show that the action of the dual adjacency matrix on the eigenspaces of the adjacency matrix is block-tridiagonal, yielding a new proof of the $Q$-polynomial property.