Let $X_r=x^r+y^r+z^r$ with $x,y,z$ real. It is known that if $S_1=0$, $(*) $ $\frac{S_{m+n}}{m+n}=\frac{S_m}{m}\frac{S_n}{n}$ for $(m,n)=(2,3),(3,2),(2,5)$, or $(5,2)$. Determine all other pairs of integers $(m,n)$ if any, so that $(*)$ holds for all real numbers $x,y,z$ such that $x+y+z=0$.