  In many cases the standard perturbation approach appears to be too simple to
describe precisely the angle resolved photoemission spectrum of strongly
correlated electron system. In particular, to describe the momentum asymmetry
observed in photoemission spectra of high-Tc cuprates a phenomenological
approach based on extremely correlated Fermi-liquid model has been recently
introduced. In this paper we analyze the general structure of the Green
function of quasiparticles in strongly correlated electron systems and stress
that it is defined not only by the self-energy of Hubbard quasiparticles but
also by a strength operator. We show that the later leads to an additional odd
momentum contribution to the spectral function and alone can explain the
observed asymmetry. So, the asymmetry of the ARPES spectra can be a measure of
the strength of electron correlations in materials.
