Contribution to the theory of the spectrum of a Bose system with condensate at small momenta
Nepomnyashchii A.A., Nepomnyashchii Yu.A.
We obtain a result Σ02(0)—0 that eliminates the divergences in the derivation of a formula for the Green's function of a Bose system with a condensate at small momenta. A simple convergent diagram expression is obtained for 1/c2 (c is the speed of sound). The conditions for the applicability of calculations using a small parameter is discussed.