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Number theory Difficulty 5.4 AIME, harder Prove it North Macedonia

In the set of integers, solve the equation
x20102006=4y2009+4y2008+2007y. x^{2010} - 2006 = 4y^{2009} + 4y^{2008} + 2007y.

Solution

Lemma. If xZx \in \mathbb{Z}, then every prime divisor of x2+1x^2+1 is of the form 4k+14k+1.

Proof of the lemma. Let px2+1p \mid x^2+1. So clearly gcd(x,p)=1\gcd(x, p)=1. Then x2+10(modp)x^2+1 \equiv 0 \pmod{p}, x21(modp)x^2 \equiv -1 \pmod{p}. Hence (x2)p12(1)p12(modp)(x^2)^{\frac{p-1}{2}} \equiv (-1)^{\frac{p-1}{2}} \pmod{p} i.e. xp12(1)p12(modp)x^{\frac{p-1}{2}} \equiv (-1)^{\frac{p-1}{2}} \pmod{p}. From Fermat's theorem we get xp121(modp)x^{\frac{p-1}{2}} \equiv 1 \pmod{p}. So p12=2k\frac{p-1}{2} = 2k i.e. p=4k+1p=4k+1. The lemma is proved.

Now the given equation is equivalent to
x2010+1=4y2009+2007y+4y2008+2007 i.e. x2010+1=(4y2008+2007)(y+1). x^{2010} + 1 = 4y^{2009} + 2007y + 4y^{2008} + 2007 \text{ i.e. } x^{2010} + 1 = (4y^{2008} + 2007)(y+1).
But 4y2008+2007=4y2008+200814y^{2008} + 2007 = 4y^{2008} + 2008 - 1 is of the form 4k14k-1 which implies that it must have a prime divisor of the form 4k14k-1. Hence (x1005)2+1(x^{1005})^2 + 1 has a prime divisor of the form 4k14k-1 which contradicts the above lemma. So the equation doesn't have a solution in the set of integers.

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