Maths Olympiad Prep

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Number theory Difficulty 6.6 National olympiad Prove it

4. 173 Does the equation x2+y3=z4x^{2}+y^{3}=z^{4} in x,y,zx, y, z have prime solutions?

Solution

[Solution] Transform the original equation into
y3=(z2x)(z2+x).y^{3}=\left(z^{2}-x\right)\left(z^{2}+x\right) .

Since yy is a prime number, we get
{z2x=1z2+x=y3\left\{\begin{array}{l} z^{2}-x=1 \\ z^{2}+x=y^{3} \end{array}\right.

or {z2x=y,z2+x=y2\left\{\begin{array}{l}z^{2}-x=y, \\ z^{2}+x=y^{2}\end{array}\right..
From (1), we get x=(z1)(z+1)x=(z-1)(z+1),
thus z1=1,z=2z-1=1, z=2,
hence x=3,y3=7x=3, y^{3}=7,
which contradicts the fact that yy is a prime number, so the system of equations (1) has no prime number solutions.
From (2), we get x=(yz)(y+z)x=(y-z)(y+z),
thus yz=1,y=z+1y-z=1, y=z+1,
hence x=2z+1x=2 z+1.
Substituting into the first equation of (2) gives
z23z2=0z^{2}-3 z-2=0

This equation has no prime number solutions, so the system of equations (2) has no prime number solutions.
In conclusion, the original equation has no prime number solutions.

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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.