2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/55750The set of space-time short-distance structures which can be described through linear operators is limited to a few basic cases. These are continua, lattices and a further short-distance structure which implies an ultraviolet cut-off. Under certain conditions, these cut-off degrees of freedom can reappear as internal degrees of freedom. We review the current status of the classification and present new conjectures.10 pages, LaTeX (Proc. Erice 98)High Energy Physics - TheoryGeneral Relativity and Quantum Cosmologytext