Additional Key Words and Phrases: formal grammars, Post correspondence problem, partial correspondence problem, one-one equivalent reducibility, one-one degrees, general problem, halting problem, ambiguity problem, nonrecursively enumerable degrees, unsolvability, decision problem for a recursively enumerable set of nonnegative integers, recursively enumerable degrees
- David G. Cantor. On the ambiguity problem of Backus systems. Journal of the ACM, 9(4):477-479, October 1962.
- Dennis F. Cudia and Wilson E. Singletary. Degrees of unsolvability in formal grammars. Journal of the ACM, 15(4):680-692, October 1968.
- Seymour Ginsburg and Joseph Ullian. Ambiguity in context free languages. Journal of the ACM, 13(1):62-89, January 1966.
- J. Hartmanis and J. E. Hopcroft. Structure of undecidable problems in automata theory. In Conference Record of 1968 Ninth Annual Symposium on Switching and Automata Theory, pages 327-333, Schenectady, New York, 15-18 October 1968. IEEE.
- Donald E. Knuth. On the translation of languages from left to right. Information and Control, 8(6):607-639, December 1965.
- Ann Yasuhara. A remark on Post normal systems. Journal of the ACM, 14(1):167-171, January 1967.