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N-D fast Fourier transform

`Y = fftn(`

returns
the multidimensional
Fourier transform of an N-D array using a fast Fourier transform
algorithm. The N-D transform is equivalent to computing the 1-D transform
along each dimension of `X`

)`X`

. The output `Y`

is
the same size as `X`

.

`Y = fftn(`

truncates `X`

,`sz`

)`X`

or
pads `X`

with trailing zeros before taking the transform
according to the elements of the vector `sz`

. Each
element of `sz`

defines the length of the corresponding
transform dimensions. For example, if `X`

is a 5-by-5-by-5
array, then `Y = fftn(X,[8 8 8])`

pads each dimension
with zeros resulting in an 8-by-8-by-8 transform `Y`

.