Eratosthenes also conceived the "Sieve of Eratosthenes ", a method of identifying prime numbers.
A prime number is a natural number greater than 1 that can be divided without remainder only by itself and by 1. Natural numbers n that can be divided by a number less than n and greater than 1 are composite numbers. The Sieve of Eratosthenes identifies all prime numbers up to a given number n as follows:
2m, 3m, 4m, ...as composite. (Thus in the first run we mark all even numbers greater than 2. In the second run we mark all multiples of 3 greater than 3.)