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The purpose of this article is to improve existing lower bounds on the chromatic number χ. Let µ 1 , . . . , µ n be the eigenvalues of the adjacency matrix sorted in nonincreasing order.First, we prove the lower bound. . , n − 1. This generalizes the Hoffman lower bound which only involves the maximum and minimum eigenvalues, i.e., the case m = 1. We provide several examples for which the new bound exceeds the Hoffman lower bound.Second, we conjecture the lower bound χ ≥ 1 + S + /S − , where S + and S − are the sums of the squares of positive and negative eigenvalues, respectively. To corroborate this conjecture, we prove the weaker bound χ ≥ S + /S − . We show that the conjectured lower bound is tight for several families of graphs. We also performed various searches for a counter-example, but none was found.Our proofs rely on a new technique of converting the adjacency matrix into the zero matrix by conjugating with unitary matrices and use majorization of spectra of self-adjoint matrices.We also show that the above bounds are actually lower bounds on the normalized orthogonal rank of a graph, which is always less than or equal to the chromatic number. The normalized orthogonal rank is the minimum dimension making it possible to assign vectors with entries of modulus one to the vertices such that two such vectors are orthogonal if the corresponding vertices are connected.All these bounds are also valid when we replace the adjacency matrix A by W * A where W is an arbitrary self-adjoint matrix and * denotes the Schur product, that is, entrywise product of W and A.

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It is well known that n/(n − µ), where µ is the spectral radius of a graph with n vertices, is a lower bound for the clique number. We conjecture that µ can be replaced in this bound with √ s + , where s + is the sum of the squares of the positive eigenvalues. We prove this conjecture for various classes of graphs, including triangle-free graphs, and for almost all graphs.

One of the best known results in spectral graph theory is the following lower bound on the chromatic number due to Alan Hoffman, where µ 1 and µ n are respectively the maximum and minimum eigenvalues of the adjacency matrix: χ ≥ 1 + µ 1 / − µ n . We recently generalised this bound to include all eigenvalues of the adjacency matrix.In this paper, we further generalize these results to include all eigenvalues of the adjacency, Laplacian and signless Laplacian matrices. The various known bounds are also unified by considering the normalized adjacency matrix, and examples are cited for which the new bounds outperform known bounds.