Clarification: sorry my notation isn't perfect. I meant the finite field F_p i.e., a set of integers between {0, 1, ... p-1} where p is a prime. Basically, I am doing FFT over a finite field with k elements. Doing so requires me to first find a primitive kth root of unity in F_p, right? so, my question is how do I do it. Most descriptions of FFT assume that the primitive root is known.