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 2020-02-11, 06:43 #1 AlbenWeeks   Feb 2020 1 Posts Function that reveals primes... NOT I've probably only found something that already existed, but am posting here to find out. Let ((2^n)-2)/n = x for any positive integer n, if x is a whole number, n is prime. if x is not a whole number, n is not prime. Is this something basic that's been found before? If so can someone let me know what this is called or why it works if there's a basic reason I'm missing?
 2020-02-11, 08:59 #2 Nick     Dec 2012 The Netherlands 1,721 Posts It's Fermat's little theorem. x can be whole without n being prime however - for example, try n=341. Then look up Carmichael numbers.
 2020-02-12, 06:23 #3 CRGreathouse     Aug 2006 3×1,993 Posts What a fantastic re-discovery! As Nick said, this is Fermat's "little" theorem in base 2, a wonderful result that is very commonly used. Its counterexamples are the base-2 pseudoprimes. You've found a new world to explore.

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