Following Wikipedia, we have the next definition of discrete Fourier transform:
$ X_k = \sum _{n=0} ^{N-1} x_n e^{{-2 \pi i n k}/{N}}$, where $k$ is an integer ranging from $0$ to $N-1$.
Everything is clear enough, but I'm always stopping at such things like $i$ in this formula. Where is it coming from? What does it mean?
I am okay with $N$, because it is coming from the context. But $i$ makes me confusing.
The formula is taken from here: https://en.wikipedia.org/wiki/Cooley-Tukey_FFT_algorithm#The_radix-2_DIT_case.
Possibly my question is too obvious, but I have no diploma and even not a student. So, I am always stopping when see such "undefined" variables.