Let be integers satisfying and . How many ordered exist?
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First, note that any square is either 1 or 0 ( m o d 4 ) . Now, suppose there exists a solution with x being odd. Since the RHS is even, either y or 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 ‘ and therefore 1 + 1 + 0 ≡ 0 ( m o d 4 ) which is a contradiction.
This means that neither x , y or z can be odd, yielding ( 2 x ‘ ) 2 + ( 2 y ‘ ) 2 + ( 2 z ‘ ) 2 = 2 4 x ‘ y ‘ z ‘ and thus x ‘ 2 + y ‘ 2 + z ‘ 2 = 2 2 x ‘ y ‘ z ‘ .
Since we can repeat the same argument for x ‘ , y ‘ and z ‘ , they can only be 0 , as any other finite value can not be devided by as many powers of 2 .