/// <summary>
/// Differentiation of a one dimensional function.
/// </summary>
public static double dfdx(IFunction1D f, double x, out double abserr)
{
// adapted from gsl_diff_central
/* Construct a divided difference table with a fairly large step
* size to get a very rough estimate of f'''. Use this to estimate
* the step size which will minimize the error in calculating f'. */
int i, k;
double h = GSL_SQRT_DBL_EPSILON, a3, temp;
double[] a = new double[4], d = new double[4];
/* Algorithm based on description on pg. 204 of Conte and de Boor
* (CdB) - coefficients of Newton form of polynomial of degree 3. */
for (i = 0; i < 4; i++)
{
a[i] = x + (i - 2) * h;
d[i] = f.f(a[i]);
}
for (k = 1; k < 5; k++)
{
for (i = 0; i < 4 - k; i++)
{
d[i] = (d[i + 1] - d[i]) / (a[i + k] - a[i]);
}
}
/* Adapt procedure described on pg. 282 of CdB to find best
* value of step size. */
a3 = Math.Abs(d[0] + d[1] + d[2] + d[3]);
if (a3 < 100 * GSL_SQRT_DBL_EPSILON)
{
a3 = 100 * GSL_SQRT_DBL_EPSILON;
}
h = Math.Pow(GSL_SQRT_DBL_EPSILON / (2 * a3), 1.0 / 3.0);
if (h > 100 * GSL_SQRT_DBL_EPSILON)
{
h = 100 * GSL_SQRT_DBL_EPSILON;
}
abserr = Math.Abs(100.0 * a3 * h * h);
temp = x + h;
GetH(x, ref h, temp);
return((f.f(x + h) - f.f(x - h)) / (2 * h));
}
private static void gsl_integration_qk(int n,
double[] xgk, double[] wg, double[] wgk,
double[] fv1, double[] fv2,
IFunction1D f, double a, double b,
out double result, out double abserr, out double resabs, out double resasc, Workspace workspace)
{
double center = 0.5 * (a + b);
double half_length = 0.5 * (b - a);
double abs_half_length = (half_length);
double f_center = f.f(center);
double result_gauss = 0;
double result_kronrod = f_center * wgk[n - 1];
double result_abs = Math.Abs(result_kronrod);
double result_asc = 0;
double mean = 0, err = 0;
int j;
if (n % 2 == 0)
{
result_gauss = f_center * wg[n / 2 - 1];
}
for (j = 0; j < (n - 1) / 2; j++)
{
int jtw = j * 2 + 1; /* j=1,2,3 jtw=2,4,6 */
double abscissa = half_length * xgk[jtw];
double fval1 = f.f(center - abscissa);
double fval2 = f.f(center + abscissa);
double fsum = fval1 + fval2;
fv1[jtw] = fval1;
fv2[jtw] = fval2;
result_gauss += wg[j] * fsum;
result_kronrod += wgk[jtw] * fsum;
result_abs += wgk[jtw] * (Math.Abs(fval1) + Math.Abs(fval2));
}
for (j = 0; j < n / 2; j++)
{
int jtwm1 = j * 2;
double abscissa = half_length * xgk[jtwm1];
double fval1 = f.f(center - abscissa);
double fval2 = f.f(center + abscissa);
fv1[jtwm1] = fval1;
fv2[jtwm1] = fval2;
result_kronrod += wgk[jtwm1] * (fval1 + fval2);
result_abs += wgk[jtwm1] * (Math.Abs(fval1) + Math.Abs(fval2));
}
mean = result_kronrod * 0.5;
result_asc = wgk[n - 1] * Math.Abs(f_center - mean);
for (j = 0; j < n - 1; j++)
{
result_asc += wgk[j] * (Math.Abs(fv1[j] - mean) + Math.Abs(fv2[j] - mean));
}
/* scale by the width of the integration region */
err = (result_kronrod - result_gauss) * half_length;
result_kronrod *= half_length;
result_abs *= abs_half_length;
result_asc *= abs_half_length;
result = result_kronrod;;
resabs = result_abs;
resasc = result_asc;
abserr = rescale_error(err, result_abs, result_asc);
}
/// <summary>
/// Integration of a smooth one dimensional function. The integral is computed until the error is below either <c>epsabs</c>
/// or <c>epsrel</c>. The algorithm is provided for fast integration of smooth functions.
/// </summary>
/// <param name="f">The function to integrate</param>
/// <param name="a">The lower integration bound</param>
/// <param name="b">The upper integration bound</param>
/// <param name="epsabs">The desired absolute error</param>
/// <param name="epsrel">The desired relative error</param>
/// <param name="abserr">The estimated error of the result</param>
/// <returns>The integral of the function</returns>
public static double q(IFunction1D f, double a, double b, double epsabs, double epsrel,
out double abserr)
{
double[] fv1 = new double[5], fv2 = new double[5], fv3 = new double[5], fv4 = new double[5];
double[] savfun = new double[21]; /* array of function values which have been computed */
double res10, res21, res43, res87; /* 10, 21, 43 and 87 point results */
double result_kronrod, result, err;
double resabs; /* approximation to the integral of abs(f) */
double resasc; /* approximation to the integral of abs(f-i/(b-a)) */
double half_length = 0.5 * (b - a);
double abs_half_length = Math.Abs(half_length);
double center = 0.5 * (b + a);
double f_center = f.f(center);
int k;
if (epsabs <= 0 && (epsrel < 50 * GSL_DBL_EPSILON || epsrel < 0.5e-28))
{
abserr = 0;
if (ThrowOnErrors)
{
throw new ArgumentException("errors too small");
}
return(double.NaN);
}
/* Compute the integral using the 10- and 21-point formula. */
res10 = 0;
res21 = w21b[5] * f_center;
resabs = w21b[5] * Math.Abs(f_center);
for (k = 0; k < 5; k++)
{
double abscissa = half_length * x1[k];
double fval1 = f.f(center + abscissa);
double fval2 = f.f(center - abscissa);
double fval = fval1 + fval2;
res10 += w10[k] * fval;
res21 += w21a[k] * fval;
resabs += w21a[k] * (Math.Abs(fval1) + Math.Abs(fval2));
savfun[k] = fval;
fv1[k] = fval1;
fv2[k] = fval2;
}
for (k = 0; k < 5; k++)
{
double abscissa = half_length * x2[k];
double fval1 = f.f(center + abscissa);
double fval2 = f.f(center - abscissa);
double fval = fval1 + fval2;
res21 += w21b[k] * fval;
resabs += w21b[k] * (Math.Abs(fval1) + Math.Abs(fval2));
savfun[k + 5] = fval;
fv3[k] = fval1;
fv4[k] = fval2;
}
resabs *= abs_half_length;
{
double mean = 0.5 * res21;
resasc = w21b[5] * Math.Abs(f_center - mean);
for (k = 0; k < 5; k++)
{
resasc +=
(w21a[k] * (Math.Abs(fv1[k] - mean) + Math.Abs(fv2[k] - mean))
+ w21b[k] * (Math.Abs(fv3[k] - mean) + Math.Abs(fv4[k] - mean)));
}
resasc *= abs_half_length;
}
result_kronrod = res21 * half_length;
err = rescale_error((res21 - res10) * half_length, resabs, resasc);
/* test for convergence. */
if (err < epsabs || err < epsrel * Math.Abs(result_kronrod))
{
result = result_kronrod;
abserr = err;
return(result);
}
/* compute the integral using the 43-point formula. */
res43 = w43b[11] * f_center;
for (k = 0; k < 10; k++)
{
res43 += savfun[k] * w43a[k];
}
for (k = 0; k < 11; k++)
{
double abscissa = half_length * x3[k];
double fval = (f.f(center + abscissa) + f.f(center - abscissa));
res43 += fval * w43b[k];
savfun[k + 10] = fval;
}
/* test for convergence */
result_kronrod = res43 * half_length;
err = rescale_error((res43 - res21) * half_length, resabs, resasc);
if (err < epsabs || err < epsrel * Math.Abs(result_kronrod))
{
result = result_kronrod;
abserr = err;
return(result);
}
/* compute the integral using the 87-point formula. */
res87 = w87b[22] * f_center;
for (k = 0; k < 21; k++)
{
res87 += savfun[k] * w87a[k];
}
for (k = 0; k < 22; k++)
{
double abscissa = half_length * x4[k];
res87 += w87b[k] * (f.f(center + abscissa) + f.f(center - abscissa));
}
/* test for convergence */
result_kronrod = res87 * half_length;
err = rescale_error((res87 - res43) * half_length, resabs, resasc);
if (err < epsabs || err < epsrel * Math.Abs(result_kronrod))
{
result = result_kronrod;
abserr = err;
return(result);
}
/* failed to converge */
result = result_kronrod;
abserr = err;
return(result);
}
