1. Some questions of phrase structure grammars I;Abraham;Computational Linguistics,1965
2. Indexed grammars—an extension of context-free grammars;Aho,1967
3. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik Sprachwissen. Kommunikat.,1961
4. Computability and Unsolvability;Davis,1958
5. The Mathematical Theory of Context-Free Languages;Ginsburg,1966