2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/27521I derive new sum rules for the electronic oscillator strengths in a periodic or nearly periodic potential, which apply within a single energy band and between any two bands. The physical origin of these sum rules is quite unlike that of conventional sum rules, and is shown to be associated with a solid-state counterpart to the principle of spectroscopic stability known in atomic physics.4 pages, RevtexCondensed Mattertext