On sentential forms of context-free grammars

Summary There is no algorithm for deciding whether two linear context-free grammars generate the same sentential forms. The equivalence problem for propagatingOL-systems is undecidable. The finiteness problem forOL-systems is decidable.SF-languages, i.e., languages which equal the set of sentential forms of a context-free grammar, possess some of the properties of context-free languages but their family is not closed under any of the ordinary operations.