Abstract
The elementary excitation in liquid □He are studied using theBrillouin-Wigner resturbation theory. Series expansion of theexcitation spectrum is studied in the Jackson-Feenberg's procedure usingthe optimum Bijl-Dingle-Jastrow tyne wave function in conjunction withthe comelated basis function. In the long wavelength limit, the seriescontains; (1) two single-ring diagrams including the leading correctionto the comolution approximation, (2) four coupled two-ring disgramswhich contribute dominantly for the fourth-order perturbation and (3)one uncoupled two-ring diagram which gives rise primarily to the k□-term takes only 1.71﹪ part in the Feynman phonon excitation. In thenumerical analysis. We have used the liquid structure function obtainedby Campbell and Feenberg in the hynerneteed-Chain approxmation.