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GfsOutputSpectra outputs the Fast Fourier Transform of a given variable. The syntax is as follows:

[ GfsOutput ] NAME {
  x = XC y = YC z = ZC 
  Lx = LX Ly = LY Lz = LZ


is the variable on which to apply the FFT.
(XC, YC, ZC) 
is the center of the line (in 1D), square (in 2D) or cube (in 3D) where the FFT is applied.
(LX, LY, LZ) 
is the length of the line (for 1D FFT), square (for 2D FFT) or cube (for 3D FFT).
is an optional argument that specifies the number of points in the domain where the FFT is applied (2^LEVEL points in L). By default this is set to the maximum level of refinement in the domain.

Note that by default all the lengths are set to zero, and that at least one length must be set (to perform a 1D FFT).

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