Sieve of Eratosthenes

There is no simple formula for generating the sequence of Prime numbers but a method devised many years ago by the mathematician Eratosthenes. It involves methodically eliminating the numbers that are know not to be prime until only the prime numbers remain. Begin by crossing out one as it is not a prime number (it does not have two factors, it is a square number). Two is a prime number but all of its multiples a not (they are composite numbers) so cross out all of the multiples of two but leave two as the first prime number. Next cross out all of the multiples of three except three itself. The number four and all of its multiples have already been crossed out as they are also multiples of two. Next cross out all the multiples of five except five itself. Continue this process until you have discovered as many prime numbers as you need.

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