Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Prove it Estonia

Find all pairs of integers (x,y)(x, y) that satisfy the equation y4=x(2x2+y)3y^4 = x(2x^2 + y)^3.

Solution

Answer: (z3(z1)2,z6(z1)2)(\frac{z^3(z-1)}{2}, \frac{z^6(z-1)}{2}) where zz is arbitrary integer.

Solution:
If x=0x = 0 then y=0y = 0. We now assume that x0x \neq 0.
Let d=gcd(x,y)>0d = \gcd(x, y) > 0 and x=da,y=dbx = da, y = db. Dividing the sides of the equation by d4d^4, we get b4=a(2a2d+b)3b^4 = a(2a^2d + b)^3. Thus ab4a \mid b^4. Since aa and bb are coprime, the only possibility is a=1|a| = 1 and the equation reduces to
b4=±(2d+b)3. b^4 = \pm (2d + b)^3.
As the same integer is simultaneously a cube and a fourth power, it is a twelfth power of some integer, yielding b=z3b = z^3 for some integer zz. Substituting b=z3b = z^3 into the equation and taking cube root gives z4=±(2d+z3)z^4 = \pm(2d + z^3) or, equivalently, ±z4=2d+z3\pm z^4 = 2d + z^3. Since z4z3z3<2d+z3-z^4 \le -|z^3| \le z^3 < 2d + z^3, the minus sign is not possible, therefore d=z4z32=z3(z1)2d = \frac{z^4-z^3}{2} = \frac{z^3(z-1)}{2}. We conclude that x=z3(z1)2x = \frac{z^3(z-1)}{2} and y=z6(z1)2y = \frac{z^6(z-1)}{2}, where zz is an arbitrary integer different from 00 and 11. In this case d>0d > 0 and the solution satisfies the original equation. If z=0z = 0 or z=1z = 1, we get the initial solution (0,0)(0, 0).

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