We show that Hoffman's sum of eigenvalues bound for the chromatic number is at least as good as the Lov\'asz theta number, but no better than the ceiling of the fractional chromatic number. In order to do so, we display an interesting connection between this sum of eigenvalues bound and a generalization of the Lov\'asz theta number introduced by Manber and Narasimhan in 1988.
Categories: math.CO math.OC