Phonon Excitations in Liquid He II

Recent experimental data on He II have been analyzed in order to test various analytic properties of the phonon dispersion curve. In particular ${\left[\frac{\epsilon \mathrm{}\left(p\right)\mathrm{}}{p}\right]}^2$, where $\epsilon \mathrm{}\left(p\right)\mathrm{}$ is the energy of an elementary excitation, seems to be well represented by a power expansion in $p$ up to $p=2.6\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$.