fast Fourier transform

there is fast Fourier transform library in Arduino and if it is available how I can use it, please?

What did Google tell you?

It's also listed in the official Arduino libraries, under the heading "Audio and Waveforms:"
https://www.arduino.cc/en/Reference/Libraries

There are many FFT implementations for Arduino.

jremington:
There are many FFT implementations for Arduino.

... and all of them come with instructions...