  Motivated by the experimental search for Bose condensation of quasiparticles
in semiconductors, the response functions of a weakly interacting Bose gas,
with isotropic but non-quadratic dispersion, are considered. Non-quadratic
dispersion modifies the definition of particle current, and leads to modified
sum rules for the current-current response function. The effect of these
modifications on the Berezhinski-Kosterlitz-Thouless transition is discussed.
