  Analytical treatment of black hole accretion generally presumes the stability
of the stationary configuration. Various authors in the past several decades
demonstrated the validity of such an assumption for inviscid hydrodynamic flow.
Inviscid assumption is a reasonable approximation for low angular^M momentum
advection dominated flow in connection to certain^M supermassive black holes at
the^M centres of the galaxies (including our own) fed from a number of stellar
donors.^M Introduction of a weak viscosity, however, may sometimes provide a
more detail understanding of the observed spectrum. Recently it has been
demonstrated that introduction of small amount of viscosity in the form of
quasi-viscous flow makes a stationary accretion disc -- where the geometric
configuration of matter is described by axisymmetric flow in hydrostatic
equilibrium -- unstable. We perform similar analysis for other disc models (for
all three possible geometric configurations of matter) for quasi-viscous models
under the post-Newtonian scheme. We introduced perturbations on the stationary
flow solution particularly in standing wave form and studied their time
evolution to observe whether they grow with time. Our analysis shows that same
sort of secular instability exists in other disc models too. We further argued
that with sufficiently low value of viscosity in the realistic astrophysical
perspective, the instability does not effectively jeopardize the stationary
condition.

High Energy Astrophysical Phenomena