For every two-dimensional system with helical boundary
conditions, there is an isomorphic one-dimensional system.
Therefore, the one-dimensional FFT of a 2-D function wrapped on a
helix is equivalent to a 2-D FFT.
We show that the Fourier dual of helical boundary conditions is
helical boundary conditions but with axes transposed, and we
explicitly link the wavenumber vector,
![${\bf k}$](img1.png)
, in a
multi-dimensional system with the wavenumber of a helical 1-D FFT,
![$k_h$](img2.png)
.
We illustrated the concepts with an example of multi-dimensional
multiple prediction.