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function [A, keep, ii, jj, xx] = mesh3d1 (n)
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% create an n-by-n-by-n 3D mesh for the 2nd difference operator
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% A = mesh3d1 (10) ; % a 10-by-10-by-10 mesh
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% Copyright 2006-2007, Timothy A. Davis.
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% http://www.cise.ufl.edu/research/sparse
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ii = zeros (7*n^3, 1) ;
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jj = zeros (7*n^3, 1) ;
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xx = zeros (7*n^3, 1) ;
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s = k*n^2 + j*n+i + 1 ;
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(k+1)*n^2 + j*n+i ]' + 1 ;
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jj (t:t+6) = [s s s s s s s] ;
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xx (t:t+6) = [-1 -1 -1 6 -1 -1 -1] ;
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keep = find (ii >= 1 & ii <= n^3 & jj >= 1 & jj <= n^3) ;
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A = sparse (ii (keep), jj (keep), xx (keep)) ;