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The excitation spectrum of liquid Helium II is re-examined using the formalism developed by Sunakawa et a!. In this formalism, certain· divergent terms appear-a constant term and the terms involving products of two field operators. We show that it is necessary to retain them in order to obtain convergent results in pertubation calculations. Results for the energy spectrum and structure factor are derived in perturbation theory and are thus shown to be convergent in contrast to the earlierdoi:10.1143/ptp.52.1719 fatcat:qdimgr4ggjgjdggtcutixrbivu