1. On formal properties of simple phrase structure grammars;Bar-Hillel,1960
2. On certain formal properties of grammars;Chomsky;Inform. and Control,1959
3. Algebraic theory of context-free languages;Chomsky,1962
4. Computability and Unsolvability;Davis,1958
5. A variant of a recursively unsolvable problem;Post;Bull. Am. Math. Soc.,1946