Calculation of the correlation functions of a nonideal Bose gas by the method of collective variables

The method of collective variables is used to investigate the properties of a nonideal Bose system near absolute zero temperature. It is shown that the method developed previously by the author for constructing correlation functions enables one to extend the method of collective variables to systems with hard core interaction. The time dependence of the correlation functions is investigated for large and small values of the time $t$.