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/* cheb/deriv.c
*
* Copyright (C) 1996, 1997, 1998, 1999, 2000 Gerard Jungman
*
* 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, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
*/
#include <config.h>
#include <stdlib.h>
#include <gsl/gsl_math.h>
#include <gsl/gsl_errno.h>
#include <gsl/gsl_chebyshev.h>
int gsl_cheb_calc_deriv(gsl_cheb_series * deriv, const gsl_cheb_series * f)
{
const size_t n = f->order + 1;
const double con = 2.0 / (f->b - f->a);
size_t i;
if(deriv->order != f->order)
{
GSL_ERROR ("order of chebyshev series must be equal", GSL_ENOMEM);
}
/* set the other parameters in the chebyshev struct */
deriv->a = f->a;
deriv->b = f->b;
#ifdef ERR
deriv->err = n * n * f->c[n-1]; /* error in derivative is n^2 c_n */
#endif
/* FIXME: should probably set deriv->f[] as well */
deriv->c[n-1] = 0.0;
if(n > 1) {
deriv->c[n-2] = 2.0 *(n-1.0) * f->c[n-1];
for(i = n-3; i>0; i--)
deriv->c[i] = deriv->c[i+2] + 2.0 *(i+1.0) * f->c[i+1];
deriv->c[0] = deriv->c[2] + 2.0 * f->c[1];
for(i = 0 ; i<n ; i++)
deriv->c[i] *= con;
}
return GSL_SUCCESS;
}
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