NAG Library Function Document

nag_ode_bvp_coll_nlin_interp (d02tyc)

+− Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

+− 10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

1 Purpose

nag_ode_bvp_coll_nlin_interp (d02tyc) interpolates on the solution of a general two-point boundary value problem computed by nag_ode_bvp_coll_nlin_solve (d02tlc).

2 Specification

#include <nag.h>

#include <nagd02.h>

void	nag_ode_bvp_coll_nlin_interp (double x, double y[], Integer neq, Integer mmax, double rcomm[], const Integer icomm[], NagError *fail)

3 Description

nag_ode_bvp_coll_nlin_interp (d02tyc) and its associated functions (nag_ode_bvp_coll_nlin_solve (d02tlc), nag_ode_bvp_coll_nlin_setup (d02tvc), nag_ode_bvp_coll_nlin_contin (d02txc) and nag_ode_bvp_coll_nlin_diag (d02tzc)) solve the two-point boundary value problem for a nonlinear mixed order system of ordinary differential equations

\begin{array}{rcl} y_{1}^{(m_{1})} (x) & = & f_{1} (x, y_{1}, y_{1}^{(1)}, \dots, y_{1}^{(m_{1} - 1)}, y_{2}, \dots, y_{n}^{(m_{n} - 1)}) \\ y_{2}^{(m_{2})} (x) & = & f_{2} (x, y_{1}, y_{1}^{(1)}, \dots, y_{1}^{(m_{1} - 1)}, y_{2}, \dots, y_{n}^{(m_{n} - 1)}) \\ ⋮ \\ y_{n}^{(m_{n})} (x) & = & f_{n} (x, y_{1}, y_{1}^{(1)}, \dots, y_{1}^{(m_{1} - 1)}, y_{2}, \dots, y_{n}^{(m_{n} - 1)}) \end{array}

over an interval

[a, b]

subject to

p

(

> 0

) nonlinear boundary conditions at

a

and

q

(

> 0

) nonlinear boundary conditions at

b

, where

p + q = \sum_{i = 1}^{n} m_{i}

. Note that

y_{i}^{(m)} (x)

is the

m

th derivative of the

i

th solution component. Hence

y_{i}^{(0)} (x) = y_{i} (x)

. The left boundary conditions at

a

are defined as

g_{i} (z (y (a))) = 0, i = 1, 2, \dots, p,

and the right boundary conditions at

b

{\bar{g}}_{j} (z (y (b))) = 0, j = 1, 2, \dots, q,

where

y = (y_{1}, y_{2}, \dots, y_{n})

and

z (y (x)) = (y_{1} (x), y_{1}^{(1)} (x), \dots, y_{1}^{(m_{1} - 1)} (x), y_{2} (x), \dots, y_{n}^{(m_{n} - 1)} (x)) .

First, nag_ode_bvp_coll_nlin_setup (d02tvc) must be called to specify the initial mesh, error requirements and other details. Then, nag_ode_bvp_coll_nlin_solve (d02tlc) can be used to solve the boundary value problem. After successful computation, nag_ode_bvp_coll_nlin_diag (d02tzc) can be used to ascertain details about the final mesh and other details of the solution procedure, and nag_ode_bvp_coll_nlin_interp (d02tyc) can be used to compute the approximate solution anywhere on the interval

[a, b]

using interpolation.

The functions are based on modified versions of the codes COLSYS and COLNEW (see Ascher et al. (1979) and Ascher and Bader (1987)). A comprehensive treatment of the numerical solution of boundary value problems can be found in Ascher et al. (1988) and Keller (1992).

4 References

Ascher U M and Bader G (1987) A new basis implementation for a mixed order boundary value ODE solver SIAM J. Sci. Stat. Comput. 8 483–500

Ascher U M, Christiansen J and Russell R D (1979) A collocation solver for mixed order systems of boundary value problems Math. Comput. 33 659–679

Ascher U M, Mattheij R M M and Russell R D (1988) Numerical Solution of Boundary Value Problems for Ordinary Differential Equations Prentice–Hall

Grossman C (1992) Enclosures of the solution of the Thomas–Fermi equation by monotone discretization J. Comput. Phys. 98 26–32

Keller H B (1992) Numerical Methods for Two-point Boundary-value Problems Dover, New York

5 Arguments

1: x – doubleInput: On entry: $x$ , the independent variable.
Constraint: $a \leq x \leq b$ , i.e., not outside the range of the original mesh specified in the initialization call to nag_ode_bvp_coll_nlin_setup (d02tvc).
2: y[ $neq \times mmax$ ] – doubleOutput: Note: where $Y (i, j)$ appears in this document, it refers to the array element $y [j \times neq + i - 1]$ .
On exit: $Y (i, j)$ contains an approximation to $y_{i}^{(j)} (x)$ , for $i = 1, 2, \dots, neq$ and $j = 0, 1, \dots, m_{i} - 1$ . The remaining elements of y (where $m_{i} < mmax$ ) are initialized to $0.0$ .
3: neq – IntegerInput: On entry: the number of differential equations.
Constraint: $neq$ must be the same value as supplied to nag_ode_bvp_coll_nlin_setup (d02tvc).
4: mmax – IntegerInput: On entry: the maximal order of the differential equations, $\max (m_{i})$ , for $i = 1, 2, \dots, neq$ .
Constraint: $mmax$ must contain the maximum value of the components of the argument m as supplied to nag_ode_bvp_coll_nlin_setup (d02tvc).
5: rcomm[ $\dim$ ] – doubleCommunication Array: Note: the dimension, $\dim$ , of this array is dictated by the requirements of associated functions that must have been previously called. This array MUST be the same array passed as argument rcomm in the previous call to nag_ode_bvp_coll_nlin_solve (d02tlc).
On entry: this must be the same array as supplied to nag_ode_bvp_coll_nlin_solve (d02tlc) and must remain unchanged between calls.

On exit: contains information about the solution for use on subsequent calls to associated functions.
6: icomm[ $\dim$ ] – const IntegerCommunication Array: Note: the dimension, $\dim$ , of this array is dictated by the requirements of associated functions that must have been previously called. This array MUST be the same array passed as argument icomm in the previous call to nag_ode_bvp_coll_nlin_solve (d02tlc).

On entry: this must be the same array as supplied to nag_ode_bvp_coll_nlin_solve (d02tlc) and must remain unchanged between calls.

On exit: contains information about the solution for use on subsequent calls to associated functions.
7: fail – NagError *Input/Output: The NAG error argument (see Section 3.6 in the Essential Introduction).

6 Error Indicators and Warnings

NE_BAD_PARAM: On entry, argument $⟨value⟩$ had an illegal value.
NE_CONVERGENCE_SOL: The solver function did not produce any results suitable for interpolation.
NE_INT_2: On entry, $mmax = ⟨value⟩$ and $\max (m [i]) = ⟨value⟩$ .
Constraint: $mmax = \max (m [i])$ .
NE_INT_CHANGED: On entry, $neq = ⟨value⟩$ and $neq = ⟨value⟩$ in nag_ode_bvp_coll_nlin_setup (d02tvc).
Constraint: $neq = neq$ in nag_ode_bvp_coll_nlin_setup (d02tvc).
NE_INTERNAL_ERROR: An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
NE_MISSING_CALL: The solver function does not appear to have been called.
NE_REAL: On entry, $x = ⟨value⟩$ .
Constraint: $x \leq ⟨value⟩$ .
On entry, $x = ⟨value⟩$ .
Constraint: $x \geq ⟨value⟩$ .
NW_NOT_CONVERGED: The solver function did not converge to a suitable solution.
A converged intermediate solution has been used.
Interpolated values should be treated with caution.
NW_TOO_MUCH_ACC_REQUESTED: The solver function did not satisfy the error requirements.
Interpolated values should be treated with caution.

7 Accuracy

If nag_ode_bvp_coll_nlin_interp (d02tyc) returns the value

fail . code =

NE_NOERROR, the computed values of the solution components

y_{i}

should be of similar accuracy to that specified by the argument tols of nag_ode_bvp_coll_nlin_setup (d02tvc). Note that during the solution process the error in the derivatives

y_{i}^{(j)}

, for

j = 1, 2, \dots, m_{i} - 1

, has not been controlled and that the derivative values returned by nag_ode_bvp_coll_nlin_interp (d02tyc) are computed via differentiation of the piecewise polynomial approximation to

y_{i}

. See also Section 9.

8 Parallelism and Performance

nag_ode_bvp_coll_nlin_interp (d02tyc) is not threaded by NAG in any implementation.

nag_ode_bvp_coll_nlin_interp (d02tyc) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.

Please consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

If nag_ode_bvp_coll_nlin_interp (d02tyc) returns the value fail.code

= NW_NOT_CONVERGED

, then the accuracy of the interpolated values may be proportional to the quantity ermx as returned by nag_ode_bvp_coll_nlin_diag (d02tzc).

If nag_ode_bvp_coll_nlin_solve (d02tlc) returned a value for fail.code other than

fail . code =

NE_NOERROR, then nothing can be said regarding either the quality or accuracy of the values computed by nag_ode_bvp_coll_nlin_interp (d02tyc).

10 Example

The following example is used to illustrate that a system with singular coefficients can be treated without modification of the system definition. See also nag_ode_bvp_coll_nlin_solve (d02tlc), nag_ode_bvp_coll_nlin_setup (d02tvc), nag_ode_bvp_coll_nlin_contin (d02txc) and nag_ode_bvp_coll_nlin_diag (d02tzc), for the illustration of other facilities.

Consider the Thomas–Fermi equation used in the investigation of potentials and charge densities of ionized atoms. See Grossman (1992), for example, and the references therein. The equation is

y^{''} = x^{- 1 / 2} y^{3 / 2}

with boundary conditions

y (0) = 1, y (a) = 0, a > 0 .

The coefficient

x^{- 1 / 2}

implies a singularity at the left-hand boundary

x = 0

We use the initial approximation

y (x) = 1 - x / a

, which satisfies the boundary conditions, on a uniform mesh of six points. For illustration we choose

a = 1

, as in Grossman (1992). Note that in ffun and fjac (see nag_ode_bvp_coll_nlin_solve (d02tlc)) we have taken the precaution of setting the function value and Jacobian value to

0.0

in case a value of

y

becomes negative, although starting from our initial solution profile this proves unnecessary during the solution phase. Of course the true solution

y (x)

is positive for all

x < a

NAG Library Function Documentnag_ode_bvp_coll_nlin_interp (d02tyc)