1. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik, Sprachwissensch. Kommunikationsforsch.,1961
2. Context-Free Grammars and Pushdown Storage;Chomsky,1962
3. Formal properties of grammars;Chomsky,1963
4. The algebraic theory of context-free languages;Chomsky,1963
5. On computability by certain classes of restricted Turing machines;Fischer,1963