public static void monogrid_poisson_1d(int n, double a, double b, double ua, double ub,
Func <double, double> force, Func <double, double> exact, ref int it_num,
ref double[] u)
//****************************************************************************80
//
// Purpose:
//
// MONOGRID_POISSON_1D solves a 1D PDE, using the Gauss-Seidel method.
//
// Discussion:
//
// This routine solves a 1D boundary value problem of the form
//
// - U''(X) = F(X) for A < X < B,
//
// with boundary conditions U(A) = UA, U(B) = UB.
//
// The Gauss-Seidel method is used.
//
// This routine is provided primarily for comparison with the
// multigrid solver.
//
// Licensing:
//
// This code is distributed under the GNU LGPL license.
//
// Modified:
//
// 26 July 2014
//
// Author:
//
// John Burkardt
//
// Reference:
//
// William Hager,
// Applied Numerical Linear Algebra,
// Prentice-Hall, 1988,
// ISBN13: 978-0130412942,
// LC: QA184.H33.
//
// Parameters:
//
// Input, int N, the number of intervals.
//
// Input, double A, B, the endpoints.
//
// Input, double UA, UB, the boundary values at the endpoints.
//
// Input, double FORCE ( double x ), the name of the function
// which evaluates the right hand side.
//
// Input, double EXACT ( double x ), the name of the function
// which evaluates the exact solution.
//
// Output, int &IT_NUM, the number of iterations.
//
// Output, double U[N+1], the computed solution.
//
{
double d1 = 0;
int i;
//
// Initialization.
//
const double tol = 0.0001;
//
// Set the nodes.
//
double[] x = typeMethods.r8vec_linspace_new(n + 1, a, b);
//
// Set the right hand side.
//
double[] r = new double[n + 1];
r[0] = ua;
double h = (b - a) / n;
for (i = 1; i < n; i++)
{
r[i] = h * h * force(x[i]);
}
r[n] = ub;
for (i = 0; i <= n; i++)
{
u[i] = 0.0;
}
it_num = 0;
//
// Gauss-Seidel iteration.
//
for (;;)
{
it_num += 1;
GaussSeidel.gauss_seidel(n + 1, r, ref u, ref d1);
if (d1 <= tol)
{
break;
}
}
}
public static void multigrid_poisson_1d(int n, double a, double b, double ua, double ub,
Func <double, double> force, Func <double, double> exact, ref int it_num,
ref double[] u)
//****************************************************************************80
//
// Purpose:
//
// MULTIGRID_POISSON_1D solves a 1D PDE using the multigrid method.
//
// Discussion:
//
// This routine solves a 1D boundary value problem of the form
//
// - U''(X) = F(X) for A < X < B,
//
// with boundary conditions U(A) = UA, U(B) = UB.
//
// The multigrid method is used.
//
// Licensing:
//
// This code is distributed under the GNU LGPL license.
//
// Modified:
//
// 26 July 2014
//
// Author:
//
// Original FORTRAN77 version by William Hager.
// C++ version by John Burkardt.
//
// Reference:
//
// William Hager,
// Applied Numerical Linear Algebra,
// Prentice-Hall, 1988,
// ISBN13: 978-0130412942,
// LC: QA184.H33.
//
// Parameters:
//
// Input, int N, the number of intervals.
// N must be a power of 2.
//
// Input, double A, B, the endpoints.
//
// Input, double UA, UB, the boundary values at the endpoints.
//
// Input, double FORCE ( double x ), the name of the function
// which evaluates the right hand side.
//
// Input, double EXACT ( double x ), the name of the function
// which evaluates the exact solution.
//
// Output, int &IT_NUM, the number of iterations.
//
// Output, double U[N+1], the computed solution.
//
{
int i;
//
// Determine if we have enough storage.
//
int k = (int)Math.Log2(n);
if (Math.Abs(n - Math.Pow(2, k)) > double.Epsilon)
{
Console.WriteLine("");
Console.WriteLine("MULTIGRID_POISSON_1D - Fatal error!");
Console.WriteLine(" N is not a power of 2.");
return;
}
int nl = n + n + k - 2;
//
// Initialization.
//
const int it = 4;
it_num = 0;
const double tol = 0.0001;
const double utol = 0.7;
int m = n;
//
// Set the nodes.
//
double[] x = typeMethods.r8vec_linspace_new(n + 1, a, b);
//
// Set the right hand side.
//
double[] r = new double[nl];
r[0] = ua;
double h = (b - a) / n;
for (i = 1; i < n; i++)
{
r[i] = h * h * force(x[i]);
}
r[n] = ub;
double[] uu = new double[nl];
for (i = 0; i < nl; i++)
{
uu[i] = 0.0;
}
//
// L points to first entry of solution
// LL points to penultimate entry.
//
int l = 0;
int ll = n - 1;
//
// Gauss-Seidel iteration
//
double d1 = 0.0;
int j = 0;
for (;;)
{
double d0 = d1;
j += 1;
GaussSeidel.gauss_seidel(n + 1, r, ref uu, ref d1, rIndex: +l, uIndex: +l);
it_num += 1;
//
// Do at least 4 iterations at each level.
//
if (j < it)
{
}
//
// Enough iterations, satisfactory decrease, on finest grid, exit.
//
else if (d1 < tol && n == m)
{
break;
}
//
// Enough iterations, satisfactory convergence, go finer.
//
else if (d1 < tol)
{
Transfer.ctof(n + 1, uu, n + n + 1, ref uu, ucIndex: +l, ufIndex: +(l - 1 - n - n));
n += n;
ll = l - 2;
l = l - 1 - n;
j = 0;
}
//
// Enough iterations, slow convergence, 2 < N, go coarser.
//
else if (utol * d0 <= d1 && 2 < n)
{
Transfer.ftoc(n + 1, uu, r, n / 2 + 1, ref uu, ref r, ufIndex: +l, rfIndex: +l,
ucIndex: +(l + n + 1), rcIndex: +(l + n + 1));
n /= 2;
l = ll + 2;
ll = ll + n + 1;
j = 0;
}
}
for (i = 0; i < n + 1; i++)
{
u[i] = uu[i];
}
}
