Number theoryDifficulty 5.3AIME, harderProve itBulgaria
Let x>y>2022 are natural numbers such that xy+x+y is a perfect square. Is it possible for each natural number z in the interval [x+3y+1,3x+y+1] the numbers x+y+z and x2+xy+y2 to not be coprimes?
Solution
We will prove that it's not possible. Let k=xy+x+y and z=x+y+2k+1. We have y<y2+2y≤xy+x+y=k≤x2+2x<x+1. Hence k∈(y,x] and z∈(x+3y+1,3x+y+1]. It's easy to check that xy+yz+zx=(x+y+k)2 and xy+yz+zx+x+y+z=(x+y+k+1)2. Let assume that there exists a prime number p, such that p∣(x+y+z,x2+xy+y2). Then x+y≡−z(modp) and so x2+xy+y2=(x+y)2−xy≡−xy−yz−zx(modp). Hence p∣xy+yz+zx and p∣x+y+z, and therefore p∣(x+y+k)2 and p∣(x+y+k+1)2, which is impossible.
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