2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/18351The conserving approximation scheme to many-body problems was developed by Kadanoff and Baym using the functional-derivative approach. Another approach for the Hubbard model also satisfies conservation laws, but in addition it satisfies the Pauli principle and a number of sum rules. A concise formal derivation of that approach, using functional derivatives, is given in this conference paper to highlight formal analogies and differences with conserving approximations.RevTex, corrected typos, updated references, as published versionStrongly Correlated ElectronsSuperconductivitytext