1. Introduction to Formal Language Theory;Harrison,1978
2. Hypercodes in deterministic and slender OL languages;Head;Inform. Contr.,1980
3. Ordering by divisibility in abstract algebras;Higman,1952
4. On syntactic monoids with distinguished subsets;Kunze,1978
5. Semigroups and Combinatorial Applications;Lallement,1979