Comment on the article "Solitary waves and double layers in an ultra-relativistic degenerate dusty electron-positron-ion plasma" [Phys. Plasmas 19, 033705 (2012)]

More recently, N. Roy et al. [Phys. Plasmas \textbf{19}, 033705 (2012)] have investigated the occurrence of nonlinear solitary and double-layers in an ultrarelativistic dusty electron-positron-ion degenerate plasma using a Sagdeev potential method. They have considered a full parametric examination on Mach-number criteria for existence of such nonlinear excitations using the specific degeneracy limits of Chandrasekhar equation of state (EoS) for Fermi-Dirac plasmas. In this comment we point-out a misleading extension of polytropic EoS to study the Fermi-Dirac relativistically degenerate plasmas.