formula

$X(k)&=&\displaystyle\frac{1}{\sqrt{N}}\sum^{N-1}_{n=0}x(n)e^{-i\left(2\pi kn/N\right)} \\ x(n)&=&\displaystyle\frac{1}{\sqrt{N}}\sum^{N-1}_{k=0}X(k)e^{i\left(2k\pi n/N\right)}$