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Number theory Difficulty 6.2 National Olympiad Prove it Italy

Problem:

Determine all solutions (p,n)(p, n) of the equation

n3=p2p1 n^{3}=p^{2}-p-1
where pp is a prime number and nn is an integer.

Solution

Solution:

The solutions of the equation are (p,n)=(2,1)(p, n)=(2,1) and (p,n)=(37,11)(p, n)=(37,11).
We rewrite the equation in the form
p(p1)=(n+1)(n2n+1). p(p-1)=(n+1)\left(n^{2}-n+1\right) .
We first observe that for every integer nn the value of n2n+1n^{2}-n+1 is positive, so all the factors of the equation written above must be positive. We have two cases.

First case: pn+1p \mid n+1.
For some positive integer mm we have n+1=mpn+1=m p and p1=m(n2n+1)p-1=m\left(n^{2}-n+1\right). But then n2n+1p1<pn+1n^{2}-n+1 \leq p-1<p \leq n+1, so n=1n=1 and p=2p=2. Substituting into the initial equation, one verifies that indeed (2,1)(2,1) is a solution.

Second case: pn2n+1p \mid n^{2}-n+1. For some positive integer mm we have n2n+1=mpn^{2}-n+1=m p and p1=m(n+1)p-1=m(n+1). Substituting the value of pp given by the second equation into the first equation, we obtain
n2(m2+1)n(m2+m1)=0. n^{2}-\left(m^{2}+1\right) n-\left(m^{2}+m-1\right)=0 \text{.}
This last equation has integer solutions if and only if its discriminant
Δ=m4+6m2+4m3=(m2+3)2+(4m12) \Delta=m^{4}+6 m^{2}+4 m-3=\left(m^{2}+3\right)^{2}+(4 m-12)
is the square of an integer. This is certainly true if 4m12=04 m-12=0, that is, if m=3m=3. Substituting this value, we obtain n=11n=11 and p=37p=37, a solution of the initial equation (n=1n=-1 gives no acceptable solutions).
On the other hand, there are no other values of mm for which Δ\Delta is a perfect square. Indeed, it is immediate to see that if m>3m>3 then (m2+3)2<Δ<(m2+4)2\left(m^{2}+3\right)^{2}<\Delta<\left(m^{2}+4\right)^{2}; in the remaining cases, one has by direct substitution that if m=1m=1 then Δ=8\Delta=8 and if m=2m=2 then Δ=21\Delta=21.
Therefore the solutions of the equation are (p,n)=(2,1)(p, n)=(2,1) and (p,n)=(37,11)(p, n)=(37,11).

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.