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/* ode-initval2/rksubs.c
*
* Copyright (C) 2008, 2009, 2010 Tuomo Keskitalo
*
* 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, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
*/
static int
rksubs (double y[], const double h, const double y0[], const double fY[],
const double b[], const size_t stage, const size_t dim)
{
/* The final substitution step in Runge-Kutta equation:
Calculates new values y by substituting the value of step size
(h), current initial values of y (y0), function f values at
Runge-Kutta points (fY), Runge-Kutta b-coefficients (b) and
method stage (stage) into the equation
y = y0 + h * sum j=1..stage (b_j * fY_j)
dim is the number of ODEs.
*/
size_t i, j;
for (i = 0; i < dim; i++)
{
y[i] = 0.0;
for (j = 0; j < stage; j++)
y[i] += b[j] * fY[j * dim + i];
}
for (i = 0; i < dim; i++)
{
y[i] *= h;
y[i] += y0[i];
}
return GSL_SUCCESS;
}
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