1. Lallement, G.,Semigroups and Combinatorial Applications, John Wiley and Sons., New York (1979).
2. Shyr, H.J.,Disjunctive Languages on a Free Monoid, Information and Control34 (1977) 123–129.
3. Lecture Notes;H.J. Shyr,1979
4. Shyr, H.J.,Ordered Catenation and Regular Free Disjunctive Languages, Information and Control,46 (1980) 257–269.
5. Shyr, H.J. and Tseng, D.C.,Some Properties of Dense Languages, Soochow Journal of Mathematics,10 (1984) 127–131.