2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/93450We review a possible framework for (non)linear quantum theories, into which linear quantum mechanics fits as well, and discuss the notion of ``equivalence'' in this setting. Finally, we draw the attention to persisting severe problems of nonlinear quantum theories.RevTeX, 10 pages, 1 figure; uses graphicx and fancybox. also available on http://www.pt.tu-clausthal.de/preprints/asi-tpa/013-97.htmlQuantum Physicstext