$$\begin{array}{llll}\hfill R_F\left(x,y,z\right)& =2R_F\left(x+\lambda ,y+\lambda ,z+\lambda \right)& \hfill & \\ \hfill & =R_F\left(\frac{x+\lambda }{4},\frac{y+\lambda }{4},\frac{z+\lambda }{4}\right)& \hfill & \\ \hfill \lambda & =\sqrt{xy}+\sqrt{yz}+\sqrt{zx}& \hfill & \end{array}$$