2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/20713I discuss a topological relation of the conduction property of a many-particle system on a periodic lattice at zero temperature to the energy spectrum. When the particle number per unit cell is an irreducible fraction $p/q$, an insulator must have $q$ low-lying states of energy $o(1/L)$ in one dimension and of energy $o(1)$ in two dimensions, where $L$ is the linear system size.4 pages (no figures) in REVTEX; revised version with minor correctionsStrongly Correlated ElectronsStatistical Mechanicstext