  Fluctuations of the number of condensed atoms in a finite-size, weakly
interacting Bose gas confined in a box potential are investigated for
temperatures up to the critical region. The canonical partition functions are
evaluated using a recursive scheme for smaller systems, and a saddle-point
approximation for larger samples, that allows to treat realistic size systems
containing up to $N \sim 10^5$ particles. We point out the importance of
particle-number constrain and interactions between out of condensate atoms for
the statistics near the critical region. For sufficiently large systems the
crossover from the anomalous to normal scaling of the fluctuations is observed.
The excitations are described in a self-consistent way within the
Bogoliubov-Popov approximation, and the interactions between thermal atoms are
described by means of the Hartree-Fock method.

Statistical Mechanics