7Hopcrofl J E.Introduction to automata theory, languages and computation[M].[S.1.]:Addison-Weslcy Publishing Company Inc, 1979.
8Howie J M.An Introduction to semigroup theory[M].London:Ac- ademic Press, 1976.
9Bondy J A, Murty U S R.Graph theory with applications[M]. New York:American Elsevier, 1976.
10Du Ding-zhu,Cao Fcng,Hsu D F.De Bruijn digraphs,Kautz di- graphs, and their generalizations[M]//Du D Z,Hsu D F.Combina- torial Network Theory.Dordrecht:Kluwer Acad Pub1,1996:65-105.
