In number theory, **primality** is the property (of a number) of being divisible by only itself **and** 1. Such a number is called a prime number.

**Note:** 1 is not prime; instead, it is a unit.

##### Interesting facts about primes

There are infinitely many prime numbers. All natural numbers greater than or equal to 2 can be expressed uniquely as the product of prime numbers. Such a product is called the prime factorization or prime decomposition of a number.

If a prime number is congruent to 1 (modulo 4), it can be expressed as the sum of two perfect squares; e.g. 5 = 1^{2} + 2^{2}, 13 = 2^{2} + 3^{2}, 17 = 1^{2} + 4^{2}

Not surprisingly, 2 is the only even prime number.

A list of prime numbers less than 100:

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97