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Number theory Difficulty 4.7 AIME Find the answer

Find all integers nn, not necessarily positive, for which there exist positive integers a,b,ca, b, c satisfying an+bn=cna^{n}+b^{n}=c^{n}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

By Fermat's Last Theorem, we know n<3n<3. Suppose n3n \leq-3. Then an+bn=cn(bc)n+a^{n}+b^{n}=c^{n} \Longrightarrow(b c)^{-n}+ (ac)n=(ab)n(a c)^{-n}=(a b)^{-n}, but since n3-n \geq 3, this is also impossible by Fermat's Last Theorem. As a result, n<3|n|<3. Furthermore, n0n \neq 0, as a0+b0=c01+1=1a^{0}+b^{0}=c^{0} \Longrightarrow 1+1=1, which is false. We now just need to find constructions for n=2,1,1,2n=-2,-1,1,2. When n=1,(a,b,c)=(1,2,3)n=1,(a, b, c)=(1,2,3) suffices, and when n=2,(a,b,c)=n=2,(a, b, c)= (3,4,5)(3,4,5) works nicely. When n=1,(a,b,c)=(6,3,2)n=-1,(a, b, c)=(6,3,2) works, and when n=2,(a,b,c)=(20,15,12)n=-2,(a, b, c)=(20,15,12) is one example. Therefore, the working values are n=±1,±2n= \pm 1, \pm 2.

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