Please see attached image. I struggling to understand why Rs is negelected in the input impedance; should'nt the total input impedance be the parrallel combination of the capacitor impedance with the Rpi plus in series with Rs?
In the referenced equation, Rs is not referenced. The capacitance is. Clearly the signal at the base of Q1 does depend on the value of Rs -- it is in series with the input signal, and there is (at least) a capacitance to GND.
A little inspection will show that the true impedance is Rs+...