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Number theory Difficulty 4.7 AIME Prove it Romania

Determine the prime numbers pp and qq that satisfy the equality
p3+107=2q(17q+24). p^3 + 107 = 2q(17q + 24).

Solution

For q=2q = 2 we obtain p=5p = 5, which is a prime. For q3q \ge 3, reducing modulo 44, we get p33(mod4)p^3 \equiv 3 \pmod 4, which leads to p3(mod4)p \equiv 3 \pmod 4. The equation reduces to p3+125=34q2+48q+18p^3 + 125 = 34q^2 + 48q + 18, or, equivalently,
(p+5)(p25p+25)=2[q2+(4q+3)2]. (p+5)(p^2-5p+25) = 2[q^2 + (4q+3)^2].
As p25p+253(mod4)p^2 - 5p + 25 \equiv 3 \pmod 4, it follows that p25p+25p^2 - 5p + 25 must have at least one prime factor d3(mod4)d \equiv 3 \pmod 4 and, because of dq2+(4q+3)2d \mid q^2 + (4q+3)^2, it follows that dqd \mid q and d4q+3d \mid 4q+3. In conclusion, d3d \mid 3, which means that d=q=3d = q = 3 and p=7p = 7, which is a prime. The solutions to the equation are (p,q){(5,2),(7,3)}(p, q) \in \{(5, 2), (7, 3)\}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.