Number theoryDifficulty 5.5AIME, harderProve itSaudi Arabia
Find all primes p for which p2−p+1 is a perfect cube.
Solution
Write the equation p2−p+1=x3 as p(p−1)=(x−1)(x2+x+1). Because p>x, p divides x2+x+1 so x2+x+1=kp and k(x−1)=p−1, for some positive integer k. It follows that k2(x−1)+k=x2+x+1 and consider the following cases:
Case 1. If k2≤x+1, then k2(x−1)+k≤x2−1+k≤x2−1+x+1<x2+x+1, not possible.
Case 2. If k2≥x+3, then k2(x−1)+k≥(x−1)(x+3)+k≥(x−1)(x+3)+x+3=x2+2x−3+x+3>x2+x+1 not possible.
Therefore x+1<k2<x+3, hence k2=x+2. This implies (x+2)(x−1)+k=x2+x+1, that is k=3. It follows x=7 and p=19.
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Source: MathNet,
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