  We investigate the reliability of standard N-body simulations by modelling of
the well-known Hernquist halo with the help of \texttt{GADGET-2} code (which
uses the tree algorithm to calculate the gravitational force) and \texttt{ph4}
code (which uses the direct summation). Comparing the results, we find that the
core formation in the halo center (which is conventionally considered as the
first sign of numerical effects, to be specific, of the collisional relaxation)
has nothing to do with the collisional relaxation, being defined by the
properties of the tree algorithm. This result casts doubts on the universally
adopted criteria of the simulation reliability in the halo center. Though we
use a halo model, which is theoretically proved to be stationary and stable, a
sort of numerical 'violent relaxation' occurs. Its properties suggest that this
effect is highly likely responsible for the central cusp formation in
cosmological modelling of the large-scale structure, and then the 'core-cusp
problem' is no more than a technical problem of N-body simulations.
