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Number theory Difficulty 6.0 National olympiad Prove it

Prove that the equation x20064y20062006=4y2007+2007yx^{2006}-4 y^{2006}-2006=4 y^{2007}+2007 y has no solution in the set of the positive integers.

Solutions — 2

Solution 1

We assume the contrary is true. So there are xx and yy that satisfy the equation. Hence we have

x2006=4y2007+4y2006+2007y+2006x2006+1=4y2006(y+1)+2007(y+1)x2006+1=(4y2006+2007)(y+1) \begin{gathered} x^{2006}=4 y^{2007}+4 y^{2006}+2007 y+2006 \\ x^{2006}+1=4 y^{2006}(y+1)+2007(y+1) \\ x^{2006}+1=\left(4 y^{2006}+2007\right)(y+1) \end{gathered}

But 4y2006+20073(mod4)4 y^{2006}+2007 \equiv 3(\bmod 4), so x2006+1x^{2006}+1 will have at least one prime divisor of the type 4k+34 k+3. It is known (and easily obtainable by using Fermat's Little Theorem) that this is impossible.

Solution 2

1. Start with the given equation:
x20064y20062006=4y2007+2007y x^{2006} - 4y^{2006} - 2006 = 4y^{2007} + 2007y

2. Rearrange the equation to isolate x2006x^{2006}:
x2006=4y2006+4y2007+2007y+2006 x^{2006} = 4y^{2006} + 4y^{2007} + 2007y + 2006

3. Factor the right-hand side:
x2006+1=4y2006(y+1)+2007(y+1) x^{2006} + 1 = 4y^{2006}(y + 1) + 2007(y + 1)
x2006+1=(4y2006+2007)(y+1) x^{2006} + 1 = (4y^{2006} + 2007)(y + 1)

4. Consider the expression 4y2006+20074y^{2006} + 2007. Notice that:
4y2006+20073(mod4) 4y^{2006} + 2007 \equiv 3 \pmod{4}
This is because 4y20060(mod4)4y^{2006} \equiv 0 \pmod{4} and 20073(mod4)2007 \equiv 3 \pmod{4}.

5. Since 4y2006+20073(mod4)4y^{2006} + 2007 \equiv 3 \pmod{4}, there exists a prime q3(mod4)q \equiv 3 \pmod{4} such that q4y2006+2007q \mid 4y^{2006} + 2007.

6. Now, consider the prime qq and the expression x2006+1x^{2006} + 1. If q4y2006+2007q \mid 4y^{2006} + 2007, then qx2006+1q \mid x^{2006} + 1.

7. However, for a prime q3(mod4)q \equiv 3 \pmod{4}, 1-1 is not a quadratic residue modulo qq. This means that there is no integer aa such that a21(modq)a^2 \equiv -1 \pmod{q}.

8. Therefore, x2006+10(modq)x^{2006} + 1 \equiv 0 \pmod{q} implies that x20061(modq)x^{2006} \equiv -1 \pmod{q}, which is impossible because 1-1 is not a quadratic residue modulo qq.

9. Hence, there is no solution in the set of positive integers for the given equation.

\blacksquare

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.