Conserving and gapless approximations for the composite bosons in terms of the constituent fermions

A long-standing problem with the many-body approximations for interacting condensed bosons has been the dichotomy between the ``conserving'' and ``gapless'' approximations, which either obey the conservations laws or satisfy the Hugenholtz-Pines condition for a gapless excitation spectrum, in the order. It is here shown that such a dichotomy does not exist for a system of composite bosons, which form as bound-fermion pairs in the strong-coupling limit of the fermionic attraction. By starting from the constituent fermions, for which conserving approximations can be constructed for any value of the mutual attraction according to the Baym-Kadanoff prescriptions, it is shown that these approximations also result in a gapless excitation spectrum for the boson-like propagators in the broken-symmetry phase. This holds provided the corresponding equations for the fermionic single- and two-particle Green's functions are solved self-consistently.