
 sieve of Eratosthenes 
  Definition: An algorithm to find all prime numbers up to a certain N. Begin with an (unmarked) array of integers from 2 to N. The first unmarked integer, 2, is the first prime. 
  The next unmarked integer is 3, so it is prime, and we mark every third integer, i.e., 6, 9, 12, etc. Note that we mark 6, 12, 18, etc. again. 
  To optimize, when we find the prime n, we can begin marking at n², since any composite less than that is a multiple of a lesser prime, and so will have been marked earlier. 
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