It is shown to be undecidable whether an c-free finite substitution and a two-state simple E-free ngsm are equivalent on {a, b}*. Consequently, the state-minimization of such ngsm’s and the nonterminal-minimization of linear grammars having two nonterminals are undecidable. Applications to some other decision problems concerning E-free finite substitutions and linear grammars are also given. Finally it is shown that the time-equivalence problem is undecidable for propagating OL systems, i.e… CONTINUE READING