2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/27252Let $H(ε, x)$ be a finite dimensional hermitian matrix depending on the variable $x$ taking its values on a 2D manifold and changing with $ε$. If at $ε=0$ two bands are touching, we give a formula for the change of Chern number of these bands as $ε$ passes through zero.uuencoded version, 6 pages latex, 2 postscript figures includedCondensed Mattertext