In mathematics, the Kronecker sum of discrete Laplacians, named after Leopold Kronecker, is a discrete version of the separation of variables for the continuous Laplacian in a rectangular cuboid[broken anchor] domain.
are 1D discrete Laplacians in the x- and y-directions, correspondingly, and
must correspond to the case of the homogeneous Dirichlet boundary condition at end points of the x- and y-intervals, in order to generate the 2D discrete Laplacian L corresponding to the homogeneous Dirichlet boundary condition everywhere on the boundary of the rectangular domain.
Here is a sample OCTAVE/MATLAB code to compute L on the regular 10×15 2D grid: Knowing all eigenvalues and eigenvectors of the factors, all eigenvalues and eigenvectors of the Kronecker product can be explicitly calculated.
Based on this, eigenvalues and eigenvectors of the Kronecker sum can also be explicitly calculated.
The eigenvalues and eigenvectors of the standard central difference approximation of the second derivative on an interval for traditional combinations of boundary conditions at the interval end points are well known.
Combining these expressions with the formulas of eigenvalues and eigenvectors for the Kronecker sum, one can easily obtain the required answer.
Each 1D discrete Laplacian must correspond to the case of the homogeneous Dirichlet boundary condition, in order to generate the 3D discrete Laplacian L corresponding to the homogeneous Dirichlet boundary condition everywhere on the boundary.
pairs the eigenvalues and the eigenvectors, while the multi-index
determines the location of the value of every eigenvector at the regular grid.
An OCTAVE/MATLAB code http://www.mathworks.com/matlabcentral/fileexchange/27279-laplacian-in-1d-2d-or-3d is available under a BSD License, which computes the sparse matrix of the 1, 2D, and 3D negative Laplacians on a rectangular grid for combinations of Dirichlet, Neumann, and Periodic boundary conditions using Kronecker sums of discrete 1D Laplacians.
The code also provides the exact eigenvalues and eigenvectors using the explicit formulas given above.