SERIES EXPANSIONS FOR EXCITED-STATES OF QUANTUM-LATTICE MODELS

We show that by means of connected-graph expansions one can effectivel y generate exact high-order series expansions which are informative of low-lying excited states for quantum many-body systems defined on a l attice. In particular, the Fourier series coefficients of elementary e xcitation spectra are directly obtained. The numerical calculations in volved are straightforward extensions of those which have already been used to calculate series expansions for ground-state correlations and T = 0 susceptibilities in a wide variety of models.