Alexandru Mateescu Arto Salomaa Sheng Yu Factorizations of Languages and Commutativity Conditions. 339-351 2002 15 Acta Cybern. 3 http://www.inf.u-szeged.hu/kutatas/actacybernetica/vol15n3/cikk3.xml db/journals/actaC/actaC15.html#MateescuSY02