I'm reading Ahlfors, Complex Analysis, pag. 135....he's generalizing Schwarz' Lemma, which states that if $f$ is analytic in the unit disc with $f(0)=0$ then $|f(z)|\leq |z|$. He says...."still more generally we may replace condition $f(0)=0$ by an arbitrary condition $f(z_0)=w_0$ with $|z_0|
So far i understood everything. Then Ahlfors states....explicitly this inequality can be written in the form $\left|\frac{M(f(z)-w_0)}{M^2-\overline w_0f(z)}\right|\leq\left|\frac{R(z-z_0)}{R^2-\overline z_0 z}\right|$ I can't understand this last expression! Some help?