Fourier spectral representation of the Laplacian acting on a 3D data array.
Fourier spectral differentiation operators are exact on a given grid when the data array satisfies the Larmor condition on that grid.
npoints - a three-element vector specifying the number of discretization points in each dimension of the 3D cube of data that the operator will be acting on, ordered as [X Y Z]. extents - a three-element vector specifying axis extents, ordered as [X Y Z].
L - Fourier spectral Laplacian, a sparse matrix designed to act on the vectorization of the 3D data array. The dimensions of the data array are assumed to be ordered as [X Y Z].
Periodic boundary conditions.
Version 2.1, authors: Ilya Kuprov