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We ought never to allow ourselves to be persuaded of the truth of anything unless on the evidence of our own reason
– René Descartes
Modular Arithmetic

Proof: a natural number n is a prime if b^(n−1) = 1 (mod n) for every integer b between 1 and n1

A number is divisible by 3 iff sum of its digits are divisible by 3
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