1. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik, Sprachwiss. Kommunikationsforsch.,1961
2. On certain formal properties of grammars;Chomsky;Inform. Control,1959
3. Context-Free Grammars and Pushdown Storage;Chomsky,1962
4. Computability and Unsolvability;Davis,1958
5. The Theory and Applications of Pushdown Store Machines;Evey,1963