1. The algebraic theory of context-free languages;Chomsky,1963
2. On formal properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetic, Sprachwiss. Kommunikationsforsch.,1961
3. Finite automata and their decision problems;Rabin;IBM J. Research,1959
4. Regularity preserving modifications of regular expressions;Stearns;Inform. Control,1963