These provide a convenient framework for analyzing decision problems.

- An alphabet is a finite set of symbols.
- A language L over is any set of strings made up of symbols in .
- Denote
**empty string**and**empty language**. - The language of all strings over
is *.

E.g., if = {0, 1}, * = {, 0, 1, 00, 01, 10, 11, 000, ..}