Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Prove it JBMO

Problem:
Find all the integers pairs (x,y)(x, y) which satisfy the equation
x5y5=16xy x^{5}-y^{5}=16 x y

Solution

Solution:
If one of x,yx, y is 00, the other has to be 00 too, and (x,y)=(0,0)(x, y) = (0, 0) is one solution.

If xy0x y \neq 0, let d=gcd(x,y)d = \gcd(x, y) and we write x=dax = d a, y=dby = d b, a,bZa, b \in \mathbb{Z} with (a,b)=1(a, b) = 1. Then, the given equation is transformed into
d3a5d3b5=16ab d^{3} a^{5} - d^{3} b^{5} = 16 a b
So, by the above equation, we conclude that ad3b5a \mid d^{3} b^{5} and thus ad3a \mid d^{3}. Similarly bd3b \mid d^{3}. Since (a,b)=1(a, b) = 1, we get that abd3a b \mid d^{3}, so we can write d3=abrd^{3} = a b r with rZr \in \mathbb{Z}. Then, equation (1) becomes
abra5abrb5=16abr(a5b5)=16 \begin{aligned} a b r a^{5} - a b r b^{5} & = 16 a b \Rightarrow \\ r\left(a^{5} - b^{5}\right) & = 16 \end{aligned}
Therefore, the difference a5b5a^{5} - b^{5} must divide 1616. Therefore, the difference a5b5a^{5} - b^{5} must divide 1616. This means that
a5b5=±1,±2,±4,±8,±16 a^{5} - b^{5} = \pm 1, \pm 2, \pm 4, \pm 8, \pm 16
The smaller values of a5b5\left|a^{5} - b^{5}\right| are 11 or 22. Indeed, if a5b5=1\left|a^{5} - b^{5}\right| = 1 then a=±1a = \pm 1 and b=0b = 0 or a=0a = 0 and b=±1b = \pm 1, a contradiction. If a5b5=2\left|a^{5} - b^{5}\right| = 2, then a=1a = 1 and b=1b = -1 or a=1a = -1 and b=1b = 1. Then r=8r = -8, and d3=8d^{3} = -8 or d=2d = -2. Therefore, (x,y)=(2,2)(x, y) = (-2, 2).

If a5b5>2\left|a^{5} - b^{5}\right| > 2 then, without loss of generality, let a>ba > b and a2a \geq 2. Putting a=x+1a = x + 1 with x1x \geq 1, we have
a5b5=(x+1)5b5(x+1)5x5=5x4+10x3+10x2+5x+131 \begin{aligned} \left|a^{5} - b^{5}\right| & = \left|(x + 1)^{5} - b^{5}\right| \\ & \geq \left|(x + 1)^{5} - x^{5}\right| \\ & = \left|5 x^{4} + 10 x^{3} + 10 x^{2} + 5 x + 1\right| \geq 31 \end{aligned}
which is impossible. Thus, the only solutions are (x,y)=(0,0)(x, y) = (0, 0) or (2,2)(-2, 2).

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.