1. Calude, C.S., Staiger, L.: On universal computably enumerable prefix codes. Math. Struct. Comput. Sci. 19(1), 45–57 (2009)
2. Han, Y.-S., Salomaa, K., Wood, D.: Nondeterministic state complexity of basic operations for prefix-free regular languages. Fundam. Inform. 90(1–2), 93–106 (2009)
3. Han, Y.-S., Salomaa, K., Wood, D.: Operational state complexity of prefix-free regular languages. In: Automata, Formal Languages, and Related Topics - Dedicated to Ferenc Gécseg on the Occasion of his 70th Birthday, pp. 99–115 (2009)
4. Lecture Notes in Computer Science;Y-S Han,2009
5. Languages and Computation;JE Hopcroft,1979