Maths Olympiad Prep

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, 2007

Number theory Difficulty 4.4 AIME Prove it JBMO

Problem:
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.

Solution

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

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