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Any examples? Sounds like you mean the reason why one is excluded from the primes because of the fundamental theorem of arithmetic.
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No, he’s right. “For any odd prime” is a not-unheard-of expression. It is usually to rule out 2 as a trivial case which may need to be handled separately.
https://en.wikipedia.org/wiki/Fermat's_theorem_on_sums_of_two_squares
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It’s not unheard of no, but if you have to rule out two for some reason it’s because of some other arbitrary choice. In the first instance (haven’t yet looked at the second and third one) it has to do with the fact that a sum of “two” was chosen arbitrary. You can come up with other things that requires you to exclude primes up to five.
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