The stability of the sonic point in isothermal disc accretion is examined for the case in which the viscous stress has a diffusive form, rather than the form of the conventional Shakura-Sunyaev-type alpha-model. The results show that the sonic point is always a saddle and always stable against small-amplitude perturbations, in contrast to the case of the alpha-model. The results suggest that the topological type of the sonic point and the stability of the point against small-amplitude perturbations are related for a wide range of problem...
