2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/19955A sum rule has been derived for the static pair correlation function. This rule is the extension of the well-known equation that relates density fluctuation to compressibility. The obtained sum rule is applied to the Bose and Fermi ideal gases as well as BCS and Bogoliubov's models.2 pages, no figuresStatistical Mechanicstext