Hello.
Here is my task:


Here is solution:



Although solution is there, I'm stuck. Firstly, how did they find transfer function? How did they treat diodes?

 

From what I see they treat the diode as a ideal device with forward voltage drop 0.7V.
Very interesting way of using GIC (General Impedance Converters) in the circuit.
Because Zu is the impedance of an inductor L = C*R^2 = 0.484Hz.
And this L together with C at the input form a LC circuit and F = 1/(2*pi*√LC) = 7.234kHz. And this is equally to oscillation frequency.