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diff --git a/octave_packages/data-smoothing-1.3.0/ddmat.m b/octave_packages/data-smoothing-1.3.0/ddmat.m
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+## Copyright (C) 2003 Paul H. C. Eilers
+##
+## This program 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 3 of the License, or (at your option) any later
+## version.
+##
+## This program 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
+## this program; if not, see .
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{D} =} ddmat (@var{x}, @var{o})
+## Compute divided differencing matrix of order @var{o}
+##
+## @itemize @w
+## @item Input
+## @itemize @w
+## @item @var{x}: vector of sampling positions
+## @item @var{o}: order of diffferences
+## @end itemize
+## @item Output
+## @itemize @w
+## @item @var{D}: the matrix; @var{D} * Y gives divided differences of order @var{o}
+## @end itemize
+## @end itemize
+##
+## References: Anal. Chem. (2003) 75, 3631.
+##
+## @end deftypefn
+
+## corrected the recursion multiplier; JJS 2/25/08
+## added error check that x is a column vector; JJS 4/13/09
+
+function D = ddmat(x, d)
+ if (nargin != 2)
+ print_usage;
+ elseif ( !iscolumn (x) )
+ error("x should be a column vector")
+ endif
+ m = length(x);
+ if d == 0
+ D = speye(m);
+ else
+ dx = x((d + 1):m) - x(1:(m - d));
+ V = sparse(diag(1 ./ dx));
+ D = d * V * diff(ddmat(x, d - 1));
+ endif
+endfunction