Dielectric function with exact exchange contribution in the electron   liquid. II. Analytical expression
  The first-order, in terms of electron-interaction in the perturbation theory, of the proper linear response function $\Pi ({\bf k}, \omega )$ gives rise to the exchange-contribution to the dielectric function $\epsilon ({\bf k} , \omega)$ in the electron liquid. Its imaginary part, $Im \Pi_1 ({\bf k}, \omega)$, is calculated exactly. An analytical expression for $Im \Pi_1 ({\bf k}, \omega)$ is derived which after refinement has a quite simple form. 
Zhixin Qian
cond-mat.str-el