  The standard mean field slave-boson solution for the infinite-$U$ Hubbard
model is revised. A slightly modified version is proposed which includes
properly the incoherent contribution of the localized states. In contrast to
the standard mean field result, this new proposed solution defines a unique
spectral function to be used in the calculation of local and not local
quantities, and satisfies the correct thermodynamic relations. The same
approach is applied also to the mean field approximation in terms of Hubbard
operators. As a byproduct of this analysis, Luttinger's theorem is shown to be
fulfilled in a natural way.
