  We study the photoionization of the $s$ states in the systems bound by
sufficiently weak central fields $V(r)$ for the large photon energies
corresponding to the relativistic photoelectrons. We demonstrate that the
energy dependence of the photoionization cross section can be obtained without
solving the wave equation. We show that the shape of the energy dependence of
the cross section is determined by analytical properties of the binding
potential $V(r)$. We find the cross sections for the potentials $V(r)$ which
have singularities in the origin, on the real axis and in the complex plane.
