I'm trying to verify that if $\phi(x_1, \cdots x_n, x_{n+1}) = \left(\frac{x_1}{1-x_{n+1}}, \cdots \frac{x_n}{1-x_{n+1}} \right) $ then
$\phi^{-1}(\zeta_1 ,\cdots \zeta_n) = \left(\frac{2\zeta_1}{(\zeta_1)^2 + \cdots (\zeta_n)^2 + 1}, \cdots \frac{2\zeta_n}{(\zeta_1)^2 + \cdots (\zeta_n)^2 + 1} ,\frac{(\zeta_1)^2 + \cdots (\zeta_n)^2 - 1}{(\zeta_1)^2 + \cdots (\zeta_n)^2 + 1}\right) $
I must be making a stupid computational error. Let's just look at the first coordinate. We get with $\zeta_1 = \frac{x_1}{1-x_{n+1}}$ that
$\frac{2\zeta_1}{(\zeta_1)^2 + \cdots (\zeta_n)^2 + 1} = \frac{2\frac{x_1}{1-x_{n+1}}}{(\frac{x_1}{1-x_{n+1}})^2 + \cdots (\frac{x_n}{1-x_{n+1}})^2 + 1}$
which after simplifying the bottom and cancelling gives
$\frac{2x_1(1-x_{n+1})}{x_1^2 + \cdots x_n^2 + (1-x_{n+1})^2}$
How does this simplify to $x_1$? I don't see it. So where's the mistake in computation?