2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/21797The principal fact that the pyroelectric vector $\Pb^0$ typical of polar crystals is invisible in equilibrium has been verified. Nevertheless, the general formula for $\Pb^0$ as a definite bulk quantity associated only with the inversion properties of a given charge distribution in the bulk has been obtained. Its application to ferroelectric perovskites BaTiO$_3$, PbTiO$_3$, and KNbO$_3$ is discussed.4 pages, 1 figure, 1 table, RevTeX4. Submitted to Physical Review LettersMaterials ScienceStrongly Correlated ElectronsMathematical Physicstext