2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/93344After recalling definition, monotonicity, concavity, and continuity of a channel's entropy with respect to a state (finite dimensional cases only), I introduce the roof property, a convex analytic tool, and show its use in treating an example. Full proofs and more examples will appear elsewhere. The relation (a la Benatti) to accessible information is mentioned.7 pages, latex, no figures. To be published in: Proceedings of the XXI International Colloquium on Group Theoretical Methods in Physics, Goslar 1996Quantum Physicstext