FD-DOMAINS AND RF-DISJUNCTIVE LANGUAGES
Yq Guo, Cm Reis, G. Thierrin, GUO, YQ (reprint author), LANZHOU UNIV,LANZHOU,PEOPLES R CHINA. · Lanzhou University Institutional Repository · 1989
Ⅰ. INTROOUCTIONLet X be a nonempty finite set called an alphabet, and X~* the free monoid generated by X and X~+=X~*—{1}, where 1 is the identity of X~* called the empty word over X. Elements and subsets of X~* are called words and languages over X respectively. The length of a word x which is the number of the letters occurring in x will be denoted by lg(x).