fact: Fixpoint fft1 n w p : {poly R} := if n is n1.+1 then let ev := fft1 n1 (w ^+ 2) (even_poly p) in let ov := fft1 n1 (w ^+ 2) (odd_poly p) in \sum_(j < 2 ^ n1) ((ev`_j + ov`_ j * w ^+ j) *: 'X^j + (ev`_j - ov`_ j * w ^+ j) *: 'X^(j + 2 ^ n1)) else (p`_0)%:P.
imports: mathcomp(all_ssreflect all_algebra ssrnum) digitn