Did you say infinite?

Let x , y , z x,y,z be integers satisfying x 2 + y 2 + z 2 = 2 x y z x^2 + y^2 + z^2 = 2xyz and x y z 0 xyz \ne0 . How many ordered ( x , y , z ) (x,y,z) exist?

0 12 Infinite 6 3

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1 solution

Simon Kaib
Apr 9, 2019

First, note that any square is either 1 1 or 0 ( m o d 4 ) 0 \pmod{4} . Now, suppose there exists a solution with x x being odd. Since the RHS is even, either y y or z z has to be odd and the other one even: ( 2 x 1 ) 2 + ( 2 y 1 ) 2 + ( 2 z ) 2 = 2 2 x y z (2x‘-1)^{2}+(2y‘-1)^{2}+(2z‘)^{2}=2^{2}xyz‘ and therefore 1 + 1 + 0 0 ( m o d 4 ) 1+1+0\equiv 0 \pmod{4} which is a contradiction.

This means that neither x x , y y or z z can be odd, yielding ( 2 x ) 2 + ( 2 y ) 2 + ( 2 z ) 2 = 2 4 x y z (2x‘)^{2}+(2y‘)^{2}+(2z‘)^{2}=2^{4}x‘y‘z‘ and thus x 2 + y 2 + z 2 = 2 2 x y z . x‘^{2}+y‘^{2}+z‘^{2}=2^{2}x‘y‘z‘.

Since we can repeat the same argument for x x‘ , y y‘ and z z‘ , they can only be 0 0 , as any other finite value can not be devided by as many powers of 2 2 .

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