1. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik, Spachwiss. u. Kommunikationsforsch.,1961
2. Properties of regular expressions and state diagrams;Brzozowski,1962
3. On certain formal properties of grammars;Chomsky;Inform. and Control,1959
4. Operations on finite automata, extended summary;Elgot,1961
5. Quotients of context free languages;Ginsburg,1962