2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/89052It is an open problem to characterize the class of languages recognized by quantum finite automata (QFA). We examine some necessary and some sufficient conditions for a (regular) language to be recognizable by a QFA. For a subclass of regular languages we get a condition which is necessary and sufficient. Also, we prove that the class of languages recognizable by a QFA is not closed under union or any other binary Boolean operation where both arguments are significant.18 pages, 16 figures, extends quant-ph/0001005Quantum Physicstext