2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/23723From very high accuracy reflectivity spectra, we have derived the optical conductivity and estimated the spectral weight up to various cut-off frequencies in underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi-2212). We show that, when evaluating the optical spectral weight over the full conduction band (1 eV), the kinetic energy decreases in the superconducting state, unlike in conventional BCS superconductors. As a consequence, the Ferrell-Glover-Tinkham sum rule is not satisfied up to this energy scale. This stands as a very unconventional behavior, contrasted with the overdoped Bi-2212 sample.6 pages with 3 figures included. Submitted to NATO Advanced Research Workshop Proceedings (Miami January 2004)Superconductivitytext