  A formalism for the study of highly interacting electronic systems is
presented. The proposed scheme is based on two key concepts: composite
operators and algebra constraints. Composite field operators, that naturally
appear as a consequence of interaction, are promoted to the rank of basic
fields in terms of which a perturbation formulation is set up. The formalism is
based on the use of Green's function and equation of motion method. The use of
composite operators requires a revisitation of the Green's function
formulation, where the representation is determined by means of algebra
constraints which are a manifestation at macroscopic level of the algebra rules
and symmetry properties obeyed at microscopic level.
