A prime number is a natural number of a particular kind. Any natural number is equal to 1 times itself. If the number is equal to any other natural numbers multiplied, then the number is called a composite number. The smallest composite number is 4, because 2 x 2 = 4. 1 is not a composite number. Every other number is a prime number. The prime numbers are the numbers other than 1 which are "not" equal to formula_1 (except 1 times itself). The smallest prime number is 2. The next prime numbers are 3, 5, 7, 11, and 13. There is no largest prime number. The set pf prime numbers is sometimes written as formula_2.