Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it Belarus

Find all pairs (p;q)(p; q) of prime numbers pp and qq satisfying the equality
p5+p3+2=q2q. p^{5} + p^{3} + 2 = q^{2} - q.

Solution

Answer: (p;q)=(2;7)(p; q) = (2; 7) and (p;q)=(3;17)(p; q) = (3; 17).
If p=2p = 2 or p=3p = 3, then the obtained quadratic equations with respect to qq have two prime solutions: q=7q = 7 and q=17q = 17.

Now let p>3p > 3. We rewrite the initial equation in the form p3(p2+1)=(q+1)(q2)p^3(p^2 + 1) = (q + 1)(q - 2). Note that the greatest common divisor of the numbers q+1q + 1 and q2q - 2 is equal to either 11 or 33. Since (q+1)(q2)÷p3(q + 1)(q - 2) \div p^3 and p3p \neq 3 we see that exactly one of the numbers q+1q + 1 and q2q - 2 can be divisible by pp, and so exactly one of them is divisible by p3p^3. If (q+1)÷p3(q + 1) \div p^3, then q+1p3q + 1 \geq p^3,

and if (q2)p3(q - 2) \nmid p^3, then q2p3q - 2 \ge p^3. In any case qp31q \ge p^3 - 1. Then we obtain p5+p3=(q+1)(q2)p3(p33)p^5 + p^3 = (q + 1)(q - 2) \ge p^3(p^3 - 3), so p2+1p33p^2 + 1 \ge p^3 - 3, i.e. 0p3p24=(p2)(p2+p+2)0 \ge p^3 - p^2 - 4 = (p - 2)(p^2 + p + 2), which is impossible for p>2p > 2. Therefore, there are no solutions different from the solutions mentioned above.

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