i study on my master's thesis about equivalent linear analysis background in soil and re-code it on Matlab.

i made a flowchart for this issue but i'm in trouble right now

first i assumed that i need a input time history and initial guess of G (shear module) and D (damping ratio) then with FFT and transfer function and Inverse FFT convert input data (time domain) to frequency domain and convert it again to time domain to compute maximum and effective shear strain and choose new G and D and repeat this circle again ...

now i have a problem with FFT and transfer function and inverse FFT part

i'm so confused in this section

i need one little help to understand this issue

thank you