FFTN FFTN N-Dimensional Forward FFT 

Usage

Computes the DFT of an N-dimensional numerical array along all
dimensions.  The general syntax for its use is

  y = fftn(x)

which computes the same-size FFTs for each dimension of x.
Alternately, you can specify the size vector

  y = fftn(x,dims)

where dims is a vector of sizes.  The array x is zero padded
or truncated as necessary in each dimension so that the output
is of size dims. The fftn function is implemented by a sequence
of calls to fft.