## Copyright (C) 2000  Kai Habel
##
## 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 2 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 <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} { } fnder (@var{pp}, @var{order})
## differentiate the spline in pp-form 
##
## @seealso{ppval}
## @end deftypefn

## Author:  Kai Habel <kai.habel@gmx.de>
## Date: 20. feb 2001

function dpp = fnder (pp, o)

  if (nargin < 1 || nargin > 2)
    usage ("fnder (pp [, order])");
  endif
  if (nargin < 2)
    o = 1;
  endif

  [X, P, N, K, D] = unmkpp (pp);
  c = columns (P);
  r = rows (P);

  for i = 1:o
    #pp.P = polyder (pp.P); matrix capable polyder is needed.
    P = P(:, 1:c - 1) .* kron ((c - 1):- 1:1, ones (r,1));
  endfor

  dpp = mkpp (X, P);

endfunction
