1. On formula properties of simple phrase structure grammars;Bar-Hillel;Z. Phonetik, Sprachwiss. Kommunikationsforsch.,1961
2. On certain formal properties of grammars;Chomsky;Inform. and Control,1959
3. Decision problems of phrase structure grammars;Landweber,1962
4. Linear bounded automata;Myhill;Wright Air Development Division, Tech. Note No. 60-165,1960