Let p be a prime. Prove that p divides abp−bap for all integers
a and b.
Answer
abp−bap=ab(bp−1−ap−1)
If p|ab, then p|(abp−bap) and also if p∤ab, then gcd(p,a)=gcd(p,b)=1,⇒bp−1≡ap−1≡1(modp) (by Fermat's little theorem).
This further implies that p|(bp−1−ap−1)⇒p|(abp−bap).
Q.E.D.
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