  The well-known Bogoliubov expression for the spectrum of a weakly interacting
dilute Bose gas becomes inadequate when the density or interactions strength
are increased. The corrections to the spectrum due to stronger interactions
were first considered by Beliaev (S. T. Beliaev, Sov. Phys.-JETP, 7:289,
(1958)). We revisit Beliaev's theory and consider its application to a dilute
gas with van der Waals interactions, where the scattering length may be tuned
via a Fano-Feshbach resonance. We numerically evaluate Beliaev's expression for
the excitation spectrum in the intermediate momentum regime, and we also
examine the consequences of the momentum dependence of the two-body scattering
amplitude.These results are relevant to the interpretation of a recent Bragg
spectroscopy experiment of a strongly interacting Bose gas.
