8th Annual Symposium on Switching and Automata Theory (SWAT 1967) (1967)
Oct. 18, 1967 to Oct. 20, 1967
DOI Bookmark: http://doi.ieeecomputersociety.org/10.1109/FOCS.1967.5
By generalizing the notions of a one counter machine and of a finite turn pushdown store automaton, it is possible to define an omega 2-hierarchy of families of context-free languages such that each subfamily is infinite, closed under union, concatenation and closure, homomorphism, intersection with regular sets and inverse homomorphism and reversal. Each subfamily can be characterized in a manner akin to the Chomsky-Schutzenberger characterization of the context-free languages. Whenever two subfamilies are incomparable it is undecidable whether a member of one belongs to the other.
S. A. Greibach, "An infinite hierarchy of context-free languages," 8th Annual Symposium on Switching and Automata Theory (SWAT 1967)(FOCS), Texas, 1967, pp. 32-36.