Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it Serbia

Problem:

Determine all pairs of natural numbers (x,n)(x, n) that are solutions of the equation

x3+2x+1=2n x^{3}+2x+1=2^{n}

Solution

Solution:

By checking, we find that for n2n \leqslant 2 the only solution is the pair (1,2)(1,2). Let us prove that for n3n \geqslant 3 there are no solutions.

The number xx must be odd, so x2+23(mod8)x^{2}+2 \equiv 3 \pmod{8}. Now from x(x2+2)1(mod8)x\left(x^{2}+2\right) \equiv -1 \pmod{8} it follows that x5(mod8)x \equiv 5 \pmod{8}. Moreover, since 3x(x2+2)3 \mid x\left(x^{2}+2\right) (if 3x3 \nmid x then 3x2+23 \mid x^{2}+2), it must be that 2n1(mod3)2^{n} \equiv 1 \pmod{3}, so nn is an even number.

Adding the number 2 to both sides of the equality gives
(x+1)(x2x+3)=2n+2 (x+1)\left(x^{2}-x+3\right)=2^{n}+2
Since nn is even, 2n2^{n} is a perfect square, so the number 2-2 is a quadratic residue modulo every odd prime divisor pp of the number (x+1)(x2x+3)(x+1)\left(x^{2}-x+3\right). Therefore
1=(2p)=(1p)(2p)=(1)p12(1)p218=(1)(p1)(p+5)8 1=\left(\frac{-2}{p}\right)=\left(\frac{-1}{p}\right)\left(\frac{2}{p}\right)=(-1)^{\frac{p-1}{2}}(-1)^{\frac{p^{2}-1}{8}}=(-1)^{\frac{(p-1)(p+5)}{8}}
from which it follows that pp is of the form 8k+18k+1 or 8k+38k+3. Being a product of such prime numbers, the number x2x+3x^{2}-x+3 itself must also be of that form. However, since x5(mod8)x \equiv 5 \pmod{8}, we have x2x+37(mod8)x^{2}-x+3 \equiv 7 \pmod{8}, which is a contradiction.

Therefore, the only solution of the given equation is (x,n)=(1,2)(x, n)=(1,2).

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