  We investigated the dynamical evolution of clusters of galaxies in virial
equilibrium using Fokker-Planck models and self-consistent N-body models. In
particular, we focused on the growth of a common halo, which is a cluster-wide
halo formed by matter stripped from galaxies, and the development of a central
density cusp. The Fokker-Planck models include the effects of two-body
gravitational encounters both between galaxies and between galaxies and common
halo particles. The effects of tidal mass stripping from the galaxies due to
close galaxy-galaxy encounters and accompanying dissipation of the orbital
kinetic energies of the galaxies were also taken into account in the
Fokker-Planck models. We find that the results of the Fokker-Planck models are
in excellent agreement with those of the N-body models regarding the growth of
the common halo mass and the evolution of the cluster density profiles. In the
central region of the cluster, a shallow density cusp, approximated by $\rho
(r) \propto r^{-\alpha}$ ($\alpha \sim$ 1), develops. This shallow cusp results
from the combined effects of two-body relaxation and tidal stripping. The cusp
steepness, $\alpha$, weakly depends on the relative importance of the tidal
stripping. When the effect of stripping is important, the central velocity
dispersion decreases as the central density increases and, consequently, a
shallow ($\alpha <2$) cusp is formed. In the limit of no stripping, usual
gravothermal core collapse occurs, i.e. the central velocity dispersion
increases as the central density increases with a steep ($\alpha >2$) cusp
left. We conclude from our consideration of the origin of the cusp demonstrated
here that shallow cusps should develop in real galaxy clusters.
