It is pointed out that zph≡F(-ωph-2 is generally a branch point of the inverse function of the function z=F(y)≡∫0(ωph)(ω2y)/(ω2y+1)g(ω)dω, where g(ω) is the normalized effective phonon spectral function of the system. There-fore it should be taken into account in evaluating the radius of convergence of the series for superconducting critical temperature of the system.