ILL 1967 Problem 46

If $x,y,z$ are real numbers satisfying relations $$x+y+z = 1 \textrm{and} \arctan x + \arctan y + \arctan z = \frac{\pi}{4},$$

prove that $x^{2n+1} + y^{2n+1} + z^{2n+1} = 1$ holds for all positive integers $n$.