# Questions about fast fourier transform operator

**35**Contributor II

Hello, everyone

I am using Fast Fourier Transform (FFT) operator

I applied it to my time series

But I have four questions

1. Originally, FFT will return a series of complex valued outputs for each attribute

But this operator returns just a series of real valued outputs

What is the meaning of the real valued outputs?

Is this FFT operator a cosine transform operator?

I feel like to know the exact formula for this FFT

FFT operator returns a series of outputs

2. What is the meaning of the number (length) of the series output?

3. What is the meaning of each number of the series output?

4. Also, if I check 'calculate frequency' box, it shows sample rate.

What is the meaning of the sample rate?

Thank you in advance