1. On the periodicity of wordlength in D0L languages;Ginsburg;Inform. Contr.,1974
2. On free monoids partially ordered by embedding;Haines;J. Combinatorial Theory,1969
3. Introduction to Formal Language Theory;Harrison,1978
4. Closure properties of some families of languages associated with biological systems;Herman;Inform. Contr.,1974
5. Developmental Systems and Languages;Herman,1975