  A simple model of Bose-Einstein condensation of interacting particles is
proposed. It is shown that in the condensate state the dependence of
thermodynamic quantities on the interaction constant does not allow an
expansion in powers of the coupling constant. Therefore it is impossible to
pass to the Einstein model of condensation in an ideal Bose gas by means of a
limiting passage, setting the interaction constant to zero. The account for the
interaction between particles eliminates difficulties in the description of
condensation available in the model of an ideal gas, which are connected with
fulfillment of thermodynamic relations and an infinite value of the particle
number fluctuation in the condensate phase.
