NETWORK FUNCTIONS OF TIME-VARYING CIRCUITS - Computer Methods for Analysis of Mixed-Mode Switching Circuits - page 41

The corresponding time-varying network function is obtained from the
inverse transform

3. Frequency Response of
Nonlinear Time-Varying Systems
The frequency response of nonlinear time-varying systems is obtained
from the Fourier transform of (3.17)

It was shown earlier that the Fourier transform of the 1st-order response
is given by

where denotes Fourier transform operator. The Fourier transform

of is obtained from

Continuing this process and substituting these results into (3.29) give