2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/26517We calculate the optical sum (OS) and the kinetic energy (KE) for a tight binding band in the Nearly Antiferromagnetic Fermi Liquid (NAFFL) model which has had some success in describing the electronic structure of the high $T_c$ cuprates. The interactions among electrons due to the exchange of spin fluctuations profoundly change the probability of occupation $(n_{{\bf k},σ})$ of states of momentum {\bf k} and spin $σ$ which is the central quantity in the calculations of OS and KE. Normal and superconducting states are considered as a function of temperature. Both integrals are found to depend importantly on interactions and an independent electron model is inadequate.9 Pages, 5 Figures Accepted for publication in Phys. Rev. BSuperconductivitytext