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Number theory Difficulty 5.5 AIME, harder Prove it Saudi Arabia

Find all primes pp for which p2p+1p^{2}-p+1 is a perfect cube.

Solution

Write the equation p2p+1=x3p^{2}-p+1 = x^{3} as
p(p1)=(x1)(x2+x+1). p(p-1) = (x-1)\left(x^{2}+x+1\right).
Because p>xp > x, pp divides x2+x+1x^{2}+x+1 so x2+x+1=kpx^{2}+x+1 = k p and k(x1)=p1k(x-1) = p-1, for some positive integer kk. It follows that
k2(x1)+k=x2+x+1 k^{2}(x-1) + k = x^{2} + x + 1
and consider the following cases:

Case 1. If k2x+1k^{2} \leq x+1, then
k2(x1)+kx21+kx21+x+1<x2+x+1, k^{2}(x-1) + k \leq x^{2} - 1 + k \leq x^{2} - 1 + \sqrt{x+1} < x^{2} + x + 1,
not possible.

Case 2. If k2x+3k^{2} \geq x+3, then
k2(x1)+k(x1)(x+3)+k(x1)(x+3)+x+3=x2+2x3+x+3>x2+x+1 \begin{aligned} k^{2}(x-1) + k & \geq (x-1)(x+3) + k \geq (x-1)(x+3) + \sqrt{x+3} \\ & = x^{2} + 2x - 3 + \sqrt{x+3} > x^{2} + x + 1 \end{aligned}
not possible.

Therefore x+1<k2<x+3x+1 < k^{2} < x+3, hence k2=x+2k^{2} = x+2. This implies
(x+2)(x1)+k=x2+x+1, (x+2)(x-1) + k = x^{2} + x + 1,
that is k=3k = 3. It follows x=7x = 7 and p=19p = 19.

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