1. Almeida, J.: Some algorithmic problems for pseudovarieties. Publ. Math. Debrecen??54, 531???552 (1999);
2. Proc. of Automata and Formal Languages, VIII
3. Almeida, J., Klíma, O.: New decidable upper bound of the 2nd level in the Straubing-Thérien concatenation hierarchy of star-free languages. DMTCS (2010)
4. Lecture Notes in Computer Science;M. Arfi,1987
5. Lecture Notes in Computer Science;M. Bojańczyk,2009