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Number theory Difficulty 5.7 AIME, harder Prove it JBMO

Problem:

Do there exist prime numbers pp and qq such that p2(p31)=q(q+1)p^{2}(p^{3}-1)=q(q+1)?

Solution

Solution:

Write the given equation in the form
p2(p1)(p2+p+1)=q(q+1) p^{2}(p-1)\left(p^{2}+p+1\right)=q(q+1)
First observe that it must not be p=qp=q, since in this case the left hand side of (9) is greater than its right hand side. Hence, since pp and qq are distinct primes, (9) immediately yields p2q+1p^{2} \mid q+1, that is
q=ap21 q=a p^{2}-1
for some aNa \in \mathbb{N}. Since pp and qq are both primes, by (9) we get the following cases:

Case 1: qp1q \mid p-1, that is
p=bq+1 p=b q+1
for some bNb \in \mathbb{N}. Substituting (11) into (10), and using the fact that a1a \geq 1 and b1b \geq 1, we obtain
q=a(bq+1)21(q+1)21=q2+2q q=a(b q+1)^{2}-1 \geq(q+1)^{2}-1=q^{2}+2 q
a contradiction.

Case 2: qp2+p+1q \mid p^{2}+p+1, that is
p2+p+1=bq p^{2}+p+1=b q
for some bNb \in \mathbb{N}. Substituting (10) into (12), we get
p2+p+1=b(ap21) p^{2}+p+1=b\left(a p^{2}-1\right)
If a2a \geq 2, then from (13) it follows that
p2+p+12p21 p^{2}+p+1 \geq 2 p^{2}-1
or equivalently, p+1(p1)(p+1)p+1 \geq(p-1)(p+1), that is, (p+1)(2p)0(p+1)(2-p) \geq 0. This implies that p=2p=2, and so q22+2+1=7q \mid 2^{2}+2+1=7. Hence, q=7q=7, but the pair p=2p=2 and q=7q=7 does not satisfy the equation (9).

Hence, it must be a=1a=1. Then if b3b \geq 3, (13) implies
p2+p+13(p21) p^{2}+p+1 \geq 3\left(p^{2}-1\right)
or equivalently, 4p(2p1)4 \geq p(2 p-1), which is obviously impossible.

Thus, it must be a=1a=1 and b{1,2}b \in\{1,2\}. For a=b=1a=b=1, (13) implies that p=2p=2, which by (12) again yields q=7q=7, which is impossible. Finally, for a=1a=1 and b=2b=2, (13) gives p(p1)=3p(p-1)=3, which is clearly not satisfied for any prime pp.

Hence, there do not exist prime numbers pp and qq which satisfy the given equation.

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