  We give a derivation of the dispersion law $\epsilon(p)=\hbar^2p^2/2m
+\widetilde V(p)-\widetilde V(0)$, where $\widetilde V(p)$ is the Fourier
transform of the pair interaction potential $V(r)$. (The interaction between
particles $x_i$ and $x_j$ is $V(|x_i-x_j|)$.)
