Sieve of Erastothenes

The method is simple:

  1. Write down the numbers from 1 to 100 (or any desired range)
  2. Start with two (the first prime number)
  3. Eliminate all its multiples
  4. Move on to the next prime (the next number on the list which you have not yet eliminated)
  5. Go back to step 3 and repeat as many times as necessary
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Note that anything above 10, or sqrt(100), does not eliminate any more numbers!