function [A keep ii jj xx] = mesh3d1 (n)
% create an n-by-n-by-n 3D mesh for the 2nd difference operator
% Example:
%   A = mesh3d1 (10) ;	% a 10-by-10-by-10 mesh
% See also: cs_demo

%   Copyright 2006, Timothy A. Davis.
%   http://www.cise.ufl.edu/research/sparse

ii = zeros (7*n^3, 1) ;
jj = zeros (7*n^3, 1) ;
xx = zeros (7*n^3, 1) ;
t = 1 ;
for k = 0:n-1
    for j = 0:n-1
	for i = 0:n-1
	    s = k*n^2 + j*n+i + 1 ;
	    ii (t:t+6) = [
		    (k-1)*n^2 + j*n+i 
		    k*n^2 + (j-1)*n+i 
		    k*n^2 + j*n+(i-1)
		    k*n^2 + j*n+i
		    k*n^2 + j*n+(i+1)
		    k*n^2 + (j+1)*n+i
		    (k+1)*n^2 + j*n+i ]' + 1 ;
	    jj (t:t+6) = [s s s s s s s] ;
	    xx (t:t+6) = [-1 -1 -1 6 -1 -1 -1] ;
	    t = t + 7 ;
	end
    end
end
keep = find (ii >= 1 & ii <= n^3 & jj >= 1 & jj <= n^3) ;
A = sparse (ii (keep), jj (keep), xx (keep)) ;
