- Alphabet is the finite, non-empty Set of the symbol
- Using the symbol stand for the Alphabet
String, sometime called word is a finite sequence of symbols, which choose from the Alphabet.
- Empty String
If the string has no symbol, it is empty String, use the stand for it.
- String’s lenth
The number of symbol in the the string called the lenth of the string, use the stand for the lenth of string
- The power of Alphabet
Use the exponent of the Alphabet like stand for the Set of string, which lenth is , in
The set of all strings in an Alphabet, use
If remove the in the normal Alphabet, the rest is called
\sum^+ = \sum^0\bigcup\sum^1\bigcup\sum^2\bigcup\dots