1. Some questions of phrase structure grammars;Abraham;Computational Linguistics,1965
2. The mathematical theory of context-free languages;Ginsburg,1966
3. Scattered context grammars;Greibach;J. Comput. System Sci.,1969
4. Simple matrix languages;Ibarra;Information and Control,1970
5. A hierarchy between context-free and context-sensitive languages;Kasai;J. Comput. System Sci.,1970