function A = mesh3d2 (n)
% create an n-by-n-by-n 3D mesh for the 2nd difference operator
% Example:
%   A = mesh3d2 (10) ;  % a 10-by-10-by-10 mesh
% See also: cs_demo

% CXSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved.
% SPDX-License-Identifier: LGPL-2.1+

nn = 1:n^3 ;
ii = [nn-n^2 ; nn-n ; nn-1 ; nn ; nn+1 ; nn+n ; nn+n^2] ;
jj = repmat (nn, 7, 1) ;
xx = repmat ([-1 -1 -1 6 -1 -1 -1]', 1, n^3) ;
keep = find (ii >= 1 & ii <= n^3 & jj >= 1 & jj <= n^3) ;
A = sparse (ii (keep), jj (keep), xx (keep)) ;