THE SIEVE OF ERATOSTHENES

A useful rule for making  a table of prime numbers is attributed  to another famous Greek astronomer and mathematician, Erastosthenes (c.275-194 B.C.) To find all the prime numbers not greater than n, we write all the integers from 1 to n in a convenient tabulated form; from this table we delete every second number after 2, because they all are divisible by 2, every third number after 3, because they all are divisible by 3, every fifth number after 5, and so on; the numbers which remain after all the deletions are prime numbers less than or equal to n.