Maths Olympiad Prep

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, 2022

Number theory Difficulty 5.3 AIME, harder Prove it Bulgaria

Let x>y>2022x > y > 2022 are natural numbers such that xy+x+yxy + x + y is a perfect square. Is it possible for each natural number zz in the interval [x+3y+1,3x+y+1][x + 3y + 1, 3x + y + 1] the numbers x+y+zx + y + z and x2+xy+y2x^2 + xy + y^2 to not be coprimes?

Solution

We will prove that it's not possible. Let k=xy+x+yk = \sqrt{xy + x + y} and z=x+y+2k+1z = x + y + 2k + 1. We have
y<y2+2yxy+x+y=kx2+2x<x+1. y < \sqrt{y^2 + 2y} \le \sqrt{xy + x + y} = k \le \sqrt{x^2 + 2x} < x + 1.
Hence k(y,x]k \in (y, x] and z(x+3y+1,3x+y+1]z \in (x + 3y + 1, 3x + y + 1]. It's easy to check that xy+yz+zx=(x+y+k)2xy + yz + zx = (x + y + k)^2 and xy+yz+zx+x+y+z=(x+y+k+1)2xy + yz + zx + x + y + z = (x + y + k + 1)^2. Let assume that there exists a prime number pp, such that p(x+y+z,x2+xy+y2)p\mid(x + y + z, x^2 + xy + y^2). Then x+yz(modp)x + y \equiv -z \pmod p and so x2+xy+y2=(x+y)2xyxyyzzx(modp)x^2 + xy + y^2 = (x + y)^2 - xy \equiv -xy - yz - zx \pmod p. Hence pxy+yz+zxp\mid xy + yz + zx and px+y+zp\mid x + y + z, and therefore p(x+y+k)2p\mid(x + y + k)^2 and p(x+y+k+1)2p\mid(x + y + k + 1)^2, which is impossible.

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