摘要
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).
基金
This research has been supported by the National Research Council of Canada under Grants A7877 and A7350, respectively. The Project is supported by the National Natural Science Foundation of China.
