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Number theory Difficulty 6.3 National olympiad Find the answer

4・172 Find all positive integer solutions (x,y,z,n)(x, y, z, n) of the equation
x2n+1y2n+1=xyz+22n+1x^{2 n+1}-y^{2 n+1}=x y z+2^{2 n+1}

satisfying the conditions n2,z522nn \geqslant 2, z \leqslant 5 \cdot 2^{2 n}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

[Solution] From the given equation, it is easy to see that xy>0x-y>0 and xx and yy have the same parity, so xy2x-y \geqslant 2.

When y=1,x=3y=1, x=3, from the equation we get
z=32n13(1+22n+1).z=3^{2 n}-\frac{1}{3}\left(1+2^{2 n+1}\right) .

To make z522nz \leqslant 5 \cdot 2^{2 n}, we should have
32n522n+13(1+22n+1)=(5+23)22n+13622n\begin{aligned} 3^{2 n} & \leqslant 5 \cdot 2^{2 n}+\frac{1}{3}\left(1+2^{2 n+1}\right) \\ & =\left(5+\frac{2}{3}\right) 2^{2 n}+\frac{1}{3} \leqslant 6 \cdot 2^{2 n} \end{aligned}

This yields n2n \leqslant 2. Since the problem requires n2n \geqslant 2, we have n=2n=2.
Substituting n=2n=2 into equation (1), we get z=70z=70, thus obtaining a solution to the original equation: (3,1,70,2)(3,1,70,2).

Next, we will prove that the equation has only this solution that meets the requirements.
In fact, when y=1,x4y=1, x \geqslant 4, since z522nz \leqslant 5 \cdot 2^{2 n} and n2n \geqslant 2, we have
x2n+1xyz=x(x2nz)4((42n522n)=22n+2(22n5)>22n+1+1=22n+1+y2n+1.\begin{aligned} x^{2 n+1}-x y z & =x\left(x^{2 n}-z\right) \\ & \geqslant 4\left(\left(4^{2 n}-5 \cdot 2^{2 n}\right)\right. \\ & =2^{2 n+2}\left(2^{2 n}-5\right) \\ & >2^{2 n+1}+1 \\ & =2^{2 n+1}+y^{2 n+1} . \end{aligned}

Thus, the equation has no solution in this case.
When y2y \geqslant 2, since xy2,z522n,n2x-y \geqslant 2, z \leqslant 5 \cdot 2^{2 n}, n \geqslant 2.
Therefore,
x2n+1xyzx[(y+2)2nyz]=x[y2n+4ny2n1+4n(2n1)y2n2++22nyz]>xy2n+x22n+y[4ny2n2+4n(2n1)y2n3522n]>y2n+1+22n+1+22n3y[8n+4n(2n1)40]y2n+1+22n+1\begin{aligned} & x^{2 n+1}-x y z \geqslant x\left[(y+2)^{2 n}-y z\right] \\ = & x\left[y^{2 n}+4 n y^{2 n-1}+4 n(2 n-1) y^{2 n-2}+\cdots+2^{2 n}-y z\right] \\ > & x y^{2 n}+x \cdot 2^{2 n}+y\left[4 n y^{2 n-2}+4 n(2 n-1) y^{2 n-3}-5 \cdot 2^{2 n}\right] \\ > & y^{2 n+1}+2^{2 n+1}+2^{2 n-3} y[8 n+4 n(2 n-1)-40] \\ \geqslant & y^{2 n+1}+2^{2 n+1} \end{aligned}

This means that the equation has no solution when y2y \geqslant 2.
In summary, the only solution to the original equation that meets the requirements is (3,1,70,2)(3,1,70,2).

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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.