in the theory of thick accretion discs, because accretion onto the black hole through the cusp of the equipotential surface, located in the equatorial plane, is possible due to a little overcoming of the critical equipotential surface by the surface of the disc (Paczyński mechanism). Accretion is thus driven by a violation of the hydrostatic equilibrium, rather than by viscosity of the accreting matter.[@Koz-Jar-Abr:1978:ASTRA:]

It is well known that all characteristic properties of the equipotential surfaces for a general rotation law are reflected by the equipotential surfaces of the simplest configurations with uniform distribution of the angular momentum density $\ell$, see Ref. . Moreover, these configurations are very important astrophysically, because they are marginally stable.[@Seg:1975:ASTRJ2:] Under the condition $$\ell(r,\theta) = \mathrm{const},
\label{eqv11}$$ a simple relation for the equipotential surfaces follows from Eq. (\[eqv9\]): $$W(r, \theta) = \ln U_t (r, \theta).                           \label{eqv12}$$

The equipotential surfaces $\theta = \theta(r)$ are given by the relation $$\oder{\theta}{r} =
    -\frac{\partial p/\partial r}{\partial p/\partial \theta},  \label{eqv13}$$ which for the configurations with $\ell = \mathrm{const}$ reduces to $$\oder{\theta}{r} =
    -\frac{\partial U_t/\partial r}{\partial U_t/\partial\theta}.\label{eqv14}$$ In the KdS spacetimes there is $$W \left(r, \theta \right)    = \ln \left\{ \frac{\rho}{I} \cdot
     \frac{\Delta^{1/2}_r \Delta^{1/2}_{\theta} 
                                        \sin \theta}{\left[\Delta_{\theta}
     \sin^{2}\theta 
                                        \left(r^2 + a^2 - 