For a graph matrix M, the Hoffman limit value H(M) is the limit (if it exists) of the largest eigenvalue (or, M-index, for short) of M(H(n)), where the graph H(n) is obtained by attaching a pendant edge to the cycle C(n-1) of length n-1.In spectral graph theory, M is usually either the adjacency matrix A or the Laplacian matrix L or the signless Laplacian matrix Q. The exact values of H(A) and H(L) were first determined by Hoffman and Guo, respectively. Since H(n) is bipartite for odd n, we have H(Q) = H(L). All graphs whose A-index is not greater than H(A) were completely described in the literature. In the present paper, we determine all graphs whose Q-index does not exceed H(Q). The results obtained are determinant to describe all graphs whose L-index is not greater then H(L). This is done precisely in Wang et al. [21]. (C) 2011 Elsevier Inc. All rights reserved.

### Graphs whose signless Laplacian spectral radius does not exceed the Hoffman limit value

#### Abstract

For a graph matrix M, the Hoffman limit value H(M) is the limit (if it exists) of the largest eigenvalue (or, M-index, for short) of M(H(n)), where the graph H(n) is obtained by attaching a pendant edge to the cycle C(n-1) of length n-1.In spectral graph theory, M is usually either the adjacency matrix A or the Laplacian matrix L or the signless Laplacian matrix Q. The exact values of H(A) and H(L) were first determined by Hoffman and Guo, respectively. Since H(n) is bipartite for odd n, we have H(Q) = H(L). All graphs whose A-index is not greater than H(A) were completely described in the literature. In the present paper, we determine all graphs whose Q-index does not exceed H(Q). The results obtained are determinant to describe all graphs whose L-index is not greater then H(L). This is done precisely in Wang et al. [21]. (C) 2011 Elsevier Inc. All rights reserved.
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Utilizza questo identificativo per citare o creare un link a questo documento: `https://hdl.handle.net/11570/1914582`
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