[en] For the investigation of the nuclear matter properties near the π-condensation critical point the general expressions for polarization operator PI and the Green function D of the pion in the nuclear matter are obtained taking into account the ππ-interactions. These quantities PI, D are calculated with the methods of the quantum statistics, in particular, with the finite Fermi system theory, for non-zero temperatures T not equal to 0. The contributions in D, PI of the pion field non-linear processes, that is of the pion excitation anharmonism in the medium, are obtained and investigated