  Analyzing general structure of the Green function of a strongly correlated
electron system we have shown that for the regime of strong correlations
Luttinger's theorem should be generalized in the following way: the volume of
the Fermi surface of the system of noninteracting particles is equal to volume
of the Fermi surface of the quasiparticles in the strongly correlated system
with account for the spectral weight of these quasiparticles. Hubbard and t-J
models analysis in the paramagnetic nonsuperconducting phase shows that the
generalized Luttinger's theorem is valid for these models.
