Computing the maximal signless Laplacian index among graphs of prescribed order and diameter

Abstract

A bug Bugp,r1,r2 is a graph obtained from a complete graph Kp by deleting an edge uv and attaching the paths Pr1 and Pr2 by one of their end vertices at u and v, respectively. Let Q(G) be the signless Laplacian matrix of a graph G and q1(G) be the spectral radius of Q(G). It is known that the bug B0 = Bugn-d+2[d/2],[d/2] e maximizes q1(G) among all graphs G of order n and diameter d. For a bug B of order n and diameter d, n - d is an eigenvalue of Q(B) with multiplicity n - d - 1. In this paper, we prove that remainder d + 1 eigenvalues of Q(B), among them q1(B), can be computed as the eigenvalues of a symmetric tridiagonal matrix of order d+1. Finally, we show that q1(B0) can be computed as the largest eigenvalue of a symmetric tridiagonal matrix of order d/2 + 1 whenever d is even.