摘要
Let ∑~* be the free monoid generated by (finite) alphabet ∑. An dement of ∑ is called a letter, an dement of ∑~* and a subset of ∑~* are respectively called a word and a language. The identity of ∑~* is called an empty word which is denoted by A; we use the notation ∑^+=∑~*-{A}. For any word x, we define lg(x) as the number of letters in x, which is called the length of x.
基金
Proiect supported by the National Natural Science Foundation of China
