A prime number is any integer greater than one which has only itself and one as factors. For example, 5 is prime since it does not have 2, 3, or 4 as a factor. 1 and 5 are the only factors of 5. Prime numbers have been studied since antiquity. Eratosthenes of Cyrene lived about 276 BCE to 194 BCE. He developed a method of finding prime numbers that is still taught today. It is called the Sieve of Eratosthenes. Prime numbers continue to fascinate mathematicians today. Prime numbers are used in number theory, and in cryptography.