Eratosthenes

Eratosthenes (ca. 276 B.C.–ca. 195 B.C.) was a Greek mathematician and astronomer. He is best known today for his calculation of the circumference of the Earth.

The sieve of Eratosthenes is a mathematical algorithm for finding all prime numbers up to a certain value, say N. After first writing down all the numbers up to N, every multiple of 2 can then be crossed out, then every multiple of 3, 5, 7, 11, and so on for every prime number less than or equal to the square root of N. In this way, composite numbers are filtered or sifted out of the written list of numbers, leaving only prime numbers remaining.