public static double p18_exact(ref p18Data data)
//****************************************************************************80
//
// Purpose:
//
// P18_EXACT returns the exact integral for problem 18.
//
// Licensing:
//
// This code is distributed under the GNU LGPL license.
//
// Modified:
//
// 27 December 2011
//
// Author:
//
// John Burkardt
//
// Parameters:
//
// Output, double P18_EXACT, the value of the integral.
//
{
double exact = Math.Pow(2.0, 3.0 * data.beta + 1.0);
return(exact);
}
public static double[] p18_fun(ref p18Data data, int n, double[] x)
//****************************************************************************80
//
// Purpose:
//
// P18_FUN evaluates the integrand for problem 18.
//
// Integral:
//
// Integral ( 0 <= x < +oo ) x^2 * exp ( - x / 2^beta ) dx
//
// Licensing:
//
// This code is distributed under the GNU LGPL license.
//
// Modified:
//
// 27 December 2011
//
// Author:
//
// John Burkardt
//
// Reference:
//
// Robert Piessens, Elise de Doncker-Kapenga,
// Christian Ueberhuber, David Kahaner,
// QUADPACK: A Subroutine Package for Automatic Integration,
// Springer, 1983, page 84.
//
// Parameters:
//
// Input, int N, the number of evaluation points.
//
// Input, double X[N], the evaluation points.
//
// Output, double P18_FUN[N], the integrand values.
//
{
int i;
double[] fx = new double[n];
for (i = 0; i < n; i++)
{
fx[i] = x[i] * x[i] * Math.Exp(-x[i] / Math.Pow(2, data.beta));
}
return(fx);
}