  In the framework of constrained systems, we give the classical Hamiltonian
formulation of slave-particle models and their correct quantization. The
electron-momentum distribution function in the t-J and Hubbard models is then
studied in the framework of slave-particle approaches and within the decoupling
scheme. We show that criticisms that have been addressed in this context coming
from a violation of the sum rule for the physical electron are not valid. Due
to the correct quantization rules for the slave-particles, the sum rule for the
physical electron is indeed obeyed, both exactly and within the decoupling
scheme.
