  Brillouin scattering spectra of the molecular glassformer propylene carbonate
(PC) in the temperature range 140 K to 350 K were analyzed using both the
phenomenological Cole-Davidson memory function and a hybrid memory function
consisting of the Cole-Davidson function plus a power-law term representing the
critical decay part of the fast beta relaxation. The spectra were also analyzed
using the extended two-correlator schematic MCT model recently employed by
Gotze and Voigtmann to analyze depolarized light backscattering,dielectric, and
neutron-scattering spectra of PC [W.Gotze and Th. Voigtman, Phys. Rev. E61,
4133 (2000). We assess the ability of the phenomenological and MCT fits, each
with three free fitting parameters, to simultaneously describe the spectra and
give reasonable values for the alpha-relaxation time tau-alpha
