formula

$R(k)&=&\displaystyle\frac{1}{\sqrt{N}}\sum^{N-1}_{n=0}x(n)\cos(2\pi kn/N) \\ I(k)&=&-\displaystyle\frac{1}{\sqrt{N}}\sum^{N-1}_{n=0}x(n)\sin(2\pi kn$