2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/28162A formulation of the Green's function method is presented in the n-pole approximation. Without referring to a specific model we give a general scheme of calculations that easily permits the computation of the "single-particle" Green's function in terms of the energy matrix. A theorem is proved which states that the moments of the spectral density function are conserved up to the order 2(n-l+1), where l is the order of the composite field. A comparison with the spectral density approach is also discussed.12 pages, RevTeXStrongly Correlated ElectronsStatistical Mechanicstext