Consider an infinite ladder network consisting of a series impedance Z and a shunt admittance Y . Let its input impedance be Z I T . If a new pair of impedance Z and admittance Y is added in front of the network, its input impedance Z I T remains unchanged since the network is infinite. Thus, it can be reduced to a finite network with one series impedance Z and two parallel impedances 1 / Y and Z IT . Its input impedance is given by the expression