Axial-symmetrical solution of the Klein-Gordon-Fock equation:
Bose-Einstein condensation

Abstract:
It is shown that axial-symmetrical solutions of the Klein-Gordon-Fock equation are classified by eigenfunctions of the wave operator that changes the sign of eigenvalues, i.e., the energy spectrum of the wave operator contains a point with a zero energy (frequency). The thermodynamics of the Bose-Einstein condensation is  discussed on the basis of the energy spectrum under consideration.