/usr/share/octave/packages/bim-1.1.5/bim3a_boundary_mass.m is in octave-bim 1.1.5-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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##
## This file is part of:
## BIM - Diffusion Advection Reaction PDE Solver
##
## BIM is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2 of the License, or
## (at your option) any later version.
##
## BIM is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with BIM; If not, see <http://www.gnu.org/licenses/>.
##
## author: Carlo de Falco <cdf _AT_ users.sourceforge.net>
## author: Matteo Porro <meoo85 _AT_ users.sourceforge.net>
## -*- texinfo -*-
## @deftypefn {Function File} {[@var{M}]} = @
## bim3a_boundary_mass(@var{mesh},@var{facelist},@var{nodelist})
##
## Build the lumped boundary mass matrix needed to apply Robin boundary
## conditions.
##
## The vector @var{facelist} contains the list of the faces contributing
## to the mass matrix.
##
## The optional argument @var{nodelist} contains the list of the
## degrees of freedom on the boundary.
##
## @seealso{bim3a_rhs, bim3a_advection_diffusion, bim3a_laplacian,
## bim3a_reaction, bim2a_boundary_mass}
## @end deftypefn
function [M] = bim3a_boundary_mass (mesh, facelist, nodelist)
## Check input
if (nargin > 3)
error ("bim3a_boundary_mass: wrong number of input parameters.");
elseif (! ((isstruct (mesh)) && (isfield (mesh, "p"))
&& (isfield (mesh, "t")) && isfield(mesh, "e")))
error (["bim3a_boundary_mass: first input", ...
" is not a valid mesh structure."]);
elseif (! ((isvector (facelist)) && (isnumeric (facelist))))
error (["bim3a_boundary_mass: second ", ...
"input is not a valid numeric vector."]);
endif
if (nargin < 3)
[nodelist] = bim3c_unknowns_on_faces (mesh, facelist);
endif
p = mesh.p;
t = [];
for ie = facelist
t = [t, mesh.e([1:3 10], mesh.e(10,:) == ie)];
endfor
area = 1/2 * norm (cross (p(:,t(2,:))-p(:,t(1,:)),
p(:,t(3,:))-p(:,t(1,:))),
2, 'columns');
dd = zeros (size (nodelist));
for in = 1:numel (nodelist)
dd (in) = 1/3 * sum (area (any (t(1:3,:) == nodelist(in))));
endfor
M = sparse (diag (dd));
endfunction
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