  An expression for the dynamic density response function has been obtained for
an interacting quantum gas in Random Phase Approximation (RPA) including first
order self and exchange contribution. It involves the single particle wave
functions and eigen values. The expression simplifies when diagonal elements
are considered. The diagonal elements of the imaginary part of Fourier
transformed response function is relevant in the measurement of Bragg
scattering cross-section and in several other applications.
