Homomorphic Images of Sentential Form Languages Defined by Semi-Thue Systems

We investigate various language famili$:s which can be obtained from sentential form languages of semi-Thue systems by applying codmgs, weak codings, non-erasing homomorphisms, and arbitrary homomorphi5rns. We also distinguish between monotone, contexi-independent, and context-dependent semi-Thur systems with at most one or finitely many starting words. Finally, we study the effect of erasing productions u -+ A. We obtain complete diagrams which show inclusions and incomparabilitiec of all the language families so defined.