  As is known, the 1/q^2 theorem of Bogoliubov asserts that the mean density of
the fermion pair states with the total momentum q obeys the inequality n_q >
C/q^2 (q \to 0) in the case of the Fermi system taken at nonzero temperature
and in the superconducting state provided the interaction term of its
Hamiltonian is locally gauge invariant. With the principle of correlation
weakening it is proved in this paper that the reason for the mentioned singular
behaviour of n_q is the presence of the bound states of particle pairs with
nonzero total momenta. Thus, below the temperature of the superconducting phase
transition there always exist the bound states of the fermion couples beyond
the pair condensate. If the pseudogap observed in the normal phase of the
high-T_c superconductors is stipulated by the presence of the electron bound
pairs, then the derived result suggests, in a model-independent manner, that
the pseudogap survives below T_c.
