NAG FL Interface
d03pef (dim1_​parab_​keller)

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

1: $\mathbf{npde}$ – Integer Input

On entry: the number of PDEs in the system to be solved.

Constraint: $\mathbf{npde}\ge 1$.

2: $\mathbf{ts}$ – Real (Kind=nag_wp) Input/Output

On entry: the initial value of the independent variable $t$.

Constraint: $\mathbf{ts}<\mathbf{tout}$.

On exit: the value of $t$ corresponding to the solution values in u. Normally $\mathbf{ts}=\mathbf{tout}$.

3: $\mathbf{tout}$ – Real (Kind=nag_wp) Input

On entry: the final value of $t$ to which the integration is to be carried out.

4: $\mathbf{pdedef}$ – Subroutine, supplied by the user. External Procedure

pdedef must compute the functions $G_i$ which define the system of PDEs. pdedef is called approximately midway between each pair of mesh points in turn by d03pef.

The specification of pdedef is:

Fortran Interface copy

Subroutine pdedef ( npde, t, x, u, ut, ux, res, ires) Integer, Intent (In) :: npde Integer, Intent (Inout) :: ires Real (Kind=nag_wp), Intent (In) :: t, x, u(npde), ut(npde), ux(npde) Real (Kind=nag_wp), Intent (Out) :: res(npde)

C Header Interface copy

void  pdedef (const Integer *npde, const double *t, const double *x, const double u[], const double ut[], const double ux[], double res[], Integer *ires)

C++ Header Interface copy

#include <nag.h> extern "C" { void  pdedef (const Integer &npde, const double &t, const double &x, const double u[], const double ut[], const double ux[], double res[], Integer &ires) }

1: $\mathbf{npde}$ – Integer Input

On entry: the number of PDEs in the system.

2: $\mathbf{t}$ – Real (Kind=nag_wp) Input

On entry: the current value of the independent variable $t$.

3: $\mathbf{x}$ – Real (Kind=nag_wp) Input

On entry: the current value of the space variable $x$.

4: $\mathbf{u}\left(\mathbf{npde}\right)$ – Real (Kind=nag_wp) array Input

On entry: $\mathbf{u}\left(\mathit{i}\right)$ contains the value of the component $U_{\mathit{i}}(x,t)$, for $\mathit{i}=1,2,\dots ,\mathbf{npde}$.

5: $\mathbf{ut}\left(\mathbf{npde}\right)$ – Real (Kind=nag_wp) array Input

On entry: $\mathbf{ut}\left(\mathit{i}\right)$ contains the value of the component $\frac{\partial U_{\mathit{i}}(x,t)}{\partial t}$, for $\mathit{i}=1,2,\dots ,\mathbf{npde}$.

6: $\mathbf{ux}\left(\mathbf{npde}\right)$ – Real (Kind=nag_wp) array Input

On entry: $\mathbf{ux}\left(\mathit{i}\right)$ contains the value of the component $\frac{\partial U_{\mathit{i}}(x,t)}{\partial x}$, for $\mathit{i}=1,2,\dots ,\mathbf{npde}$.

7: $\mathbf{res}\left(\mathbf{npde}\right)$ – Real (Kind=nag_wp) array Output

On exit: $\mathbf{res}\left(\mathit{i}\right)$ must contain the $\mathit{i}$th component of $G$, for $\mathit{i}=1,2,\dots ,\mathbf{npde}$, where $G$ is defined as

$$G_{\mathit{i}}=\sum _{\mathit{j}=1}^{\mathbf{npde}}{P}_{\mathit{i},\mathit{j}}\frac{\partial U_{\mathit{j}}}{\partial t}\text{,}$$

(8)

i.e., only terms depending explicitly on time derivatives, or

$$G_{\mathit{i}}=\sum _{\mathit{j}=1}^{\mathbf{npde}}{P}_{\mathit{i},\mathit{j}}\frac{\partial U_{\mathit{j}}}{\partial t}+Q_{\mathit{i}}\text{,}$$

(9)

i.e., all terms in equation (2).

The definition of $G$ is determined by the input value of ires.

8: $\mathbf{ires}$ – Integer Input/Output

On entry: the form of $G_i$ that must be returned in the array res.

$\mathbf{ires}=\mathrm{-1}$

Equation (8) must be used.

$\mathbf{ires}=1$

Equation (9) must be used.

On exit: should usually remain unchanged. However, you may set ires to force the integration routine to take certain actions, as described below:

$\mathbf{ires}=2$

Indicates to the integrator that control should be passed back immediately to the calling (sub)routine with the error indicator set to $\mathbf{ifail}=\mathbf{6}$.

$\mathbf{ires}=3$

Indicates to the integrator that the current time step should be abandoned and a smaller time step used instead. You may wish to set $\mathbf{ires}=3$ when a physically meaningless input or output value has been generated. If you consecutively set $\mathbf{ires}=3$, d03pef returns to the calling subroutine with the error indicator set to $\mathbf{ifail}=\mathbf{4}$.

pdedef must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which d03pef is called. Arguments denoted as Input must not be changed by this procedure.

Note: pdedef should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by d03pef. If your code inadvertently does return any NaNs or infinities, d03pef is likely to produce unexpected results.

5: $\mathbf{bndary}$ – Subroutine, supplied by the user. External Procedure

bndary must compute the functions $G_i^L$ and $G_i^R$ which define the boundary conditions as in equations (4) and (5).

The specification of bndary is:

Fortran Interface copy

Subroutine bndary ( npde, t, ibnd, nobc, u, ut, res, ires) Integer, Intent (In) :: npde, ibnd, nobc Integer, Intent (Inout) :: ires Real (Kind=nag_wp), Intent (In) :: t, u(npde), ut(npde) Real (Kind=nag_wp), Intent (Out) :: res(nobc)

C Header Interface copy

void  bndary (const Integer *npde, const double *t, const Integer *ibnd, const Integer *nobc, const double u[], const double ut[], double res[], Integer *ires)

C++ Header Interface copy

#include <nag.h> extern "C" { void  bndary (const Integer &npde, const double &t, const Integer &ibnd, const Integer &nobc, const double u[], const double ut[], double res[], Integer &ires) }

1: $\mathbf{npde}$ – Integer Input

On entry: the number of PDEs in the system.

2: $\mathbf{t}$ – Real (Kind=nag_wp) Input

On entry: the current value of the independent variable $t$.

3: $\mathbf{ibnd}$ – Integer Input

On entry: determines the position of the boundary conditions.

$\mathbf{ibnd}=0$

bndary must compute the left-hand boundary condition at $x=a$.

$\mathbf{ibnd}\ne 0$

Indicates that bndary must compute the right-hand boundary condition at $x=b$.

4: $\mathbf{nobc}$ – Integer Input

On entry: specifies the number of boundary conditions at the boundary specified by ibnd.

5: $\mathbf{u}\left(\mathbf{npde}\right)$ – Real (Kind=nag_wp) array Input

On entry: $\mathbf{u}\left(\mathit{i}\right)$ contains the value of the component $U_{\mathit{i}}(x,t)$ at the boundary specified by ibnd, for $\mathit{i}=1,2,\dots ,\mathbf{npde}$.

6: $\mathbf{ut}\left(\mathbf{npde}\right)$ – Real (Kind=nag_wp) array Input

On entry: $\mathbf{ut}\left(\mathit{i}\right)$ contains the value of the component $\frac{\partial U_{\mathit{i}}(x,t)}{\partial t}$ at the boundary specified by ibnd, for $\mathit{i}=1,2,\dots ,\mathbf{npde}$.

7: $\mathbf{res}\left(\mathbf{nobc}\right)$ – Real (Kind=nag_wp) array Output

On exit: $\mathbf{res}\left(\mathit{i}\right)$ must contain the $\mathit{i}$th component of $G^L$ or $G^R$, depending on the value of ibnd, for $\mathit{i}=1,2,\dots ,\mathbf{nobc}$, where $G^L$ is defined as

$$G_{\mathit{i}}^L=\sum _{\mathit{j}=1}^{\mathbf{npde}}{E}_{\mathit{i},\mathit{j}}^L\frac{\partial U_{\mathit{j}}}{\partial t}\text{,}$$

(10)

i.e., only terms depending explicitly on time derivatives, or

$$G_{\mathit{i}}^L=\sum _{\mathit{j}=1}^{\mathbf{npde}}{E}_{\mathit{i},\mathit{j}}^L\frac{\partial U_{\mathit{j}}}{\partial t}+S_{\mathit{i}}^L\text{,}$$

(11)

i.e., all terms in equation (6), and similarly for $G_{\mathit{i}}^R$.
The definitions of $G^L$ and $G^R$ are determined by the input value of ires.

8: $\mathbf{ires}$ – Integer Input/Output

On entry: the form $G_i^L$ (or $G_i^R$) that must be returned in the array res.

$\mathbf{ires}=\mathrm{-1}$

Equation (10) must be used.

$\mathbf{ires}=1$

Equation (11) must be used.

On exit: should usually remain unchanged. However, you may set ires to force the integration routine to take certain actions, as described below:

$\mathbf{ires}=2$

Indicates to the integrator that control should be passed back immediately to the calling (sub)routine with the error indicator set to $\mathbf{ifail}=\mathbf{6}$.

$\mathbf{ires}=3$

Indicates to the integrator that the current time step should be abandoned and a smaller time step used instead. You may wish to set $\mathbf{ires}=3$ when a physically meaningless input or output value has been generated. If you consecutively set $\mathbf{ires}=3$, d03pef returns to the calling subroutine with the error indicator set to $\mathbf{ifail}=\mathbf{4}$.

bndary must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which d03pef is called. Arguments denoted as Input must not be changed by this procedure.

Note: bndary should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by d03pef. If your code inadvertently does return any NaNs or infinities, d03pef is likely to produce unexpected results.

6: $\mathbf{u}(\mathbf{npde},\mathbf{npts})$ – Real (Kind=nag_wp) array Input/Output

On entry: the initial values of $U(x,t)$ at $t=\mathbf{ts}$ and the mesh points $\mathbf{x}\left(\mathit{j}\right)$, for $\mathit{j}=1,2,\dots ,\mathbf{npts}$.

On exit: $\mathbf{u}(\mathit{i},\mathit{j})$ will contain the computed solution at $t=\mathbf{ts}$.

7: $\mathbf{npts}$ – Integer Input

On entry: the number of mesh points in the interval $[a,b]$.

Constraint: $\mathbf{npts}\ge 3$.

8: $\mathbf{x}\left(\mathbf{npts}\right)$ – Real (Kind=nag_wp) array Input

On entry: the mesh points in the spatial direction. $\mathbf{x}\left(1\right)$ must specify the left-hand boundary, $a$, and $\mathbf{x}\left(\mathbf{npts}\right)$ must specify the right-hand boundary, $b$.

Constraint: $\mathbf{x}\left(1\right)<\mathbf{x}\left(2\right)<\cdots <\mathbf{x}\left(\mathbf{npts}\right)$.

9: $\mathbf{nleft}$ – Integer Input

On entry: the number $n_a$ of boundary conditions at the left-hand mesh point $\mathbf{x}\left(1\right)$.

Constraint: $0\le \mathbf{nleft}\le \mathbf{npde}$.

10: $\mathbf{acc}$ – Real (Kind=nag_wp) Input

On entry: a positive quantity for controlling the local error estimate in the time integration. If $E(i,j)$ is the estimated error for $U_i$ at the $j$th mesh point, the error test is:

$$\left|E(i,j)\right|=\mathbf{acc}\times (1.0+\left|\mathbf{u}(i,j)\right|)\text{.}$$

Constraint: $\mathbf{acc}>0.0$.

11: $\mathbf{rsave}\left(\mathbf{lrsave}\right)$ – Real (Kind=nag_wp) array Communication Array

If $\mathbf{ind}=0$, rsave need not be set on entry.

If $\mathbf{ind}=1$, rsave must be unchanged from the previous call to the routine because it contains required information about the iteration.

12: $\mathbf{lrsave}$ – Integer Input

On entry: the dimension of the array rsave as declared in the (sub)program from which d03pef is called.

Constraint: $$\begin{gathered} \mathbf{lrsave}\ge (4\times \mathbf{npde}+\mathbf{nleft}+14)\times \mathbf{npde}\times \mathbf{npts}+(3\times \mathbf{npde}+21)\times \mathbf{npde}+ \\ 7\times \mathbf{npts}+54 \end{gathered}$$.

13: $\mathbf{isave}\left(\mathbf{lisave}\right)$ – Integer array Communication Array

If $\mathbf{ind}=0$, isave need not be set on entry.

If $\mathbf{ind}=1$, isave must be unchanged from the previous call to the routine because it contains required information about the iteration. In particular:

$\mathbf{isave}\left(1\right)$

Contains the number of steps taken in time.

$\mathbf{isave}\left(2\right)$

Contains the number of residual evaluations of the resulting ODE system used. One such evaluation involves computing the PDE functions at all the mesh points, as well as one evaluation of the functions in the boundary conditions.

$\mathbf{isave}\left(3\right)$

Contains the number of Jacobian evaluations performed by the time integrator.

$\mathbf{isave}\left(4\right)$

Contains the order of the last backward differentiation formula method used.

$\mathbf{isave}\left(5\right)$

Contains the number of Newton iterations performed by the time integrator. Each iteration involves an ODE residual evaluation followed by a back-substitution using the $LU$ decomposition of the Jacobian matrix.

14: $\mathbf{lisave}$ – Integer Input

On entry: the dimension of the array isave as declared in the (sub)program from which d03pef is called.

Constraint: $\mathbf{lisave}\ge \mathbf{npde}\times \mathbf{npts}+24$.

15: $\mathbf{itask}$ – Integer Input

On entry: specifies the task to be performed by the ODE integrator.

$\mathbf{itask}=1$

Normal computation of output values $\mathbf{u}$ at $t=\mathbf{tout}$.

$\mathbf{itask}=2$

Take one step and return.

$\mathbf{itask}=3$

Stop at the first internal integration point at or beyond $t=\mathbf{tout}$.

Constraint: $\mathbf{itask}=1$, $2$ or $3$.

16: $\mathbf{itrace}$ – Integer Input

On entry: the level of trace information required from d03pef and the underlying ODE solver as follows:

$\mathbf{itrace}\le \mathrm{-1}$

No output is generated.

$\mathbf{itrace}=0$

Only warning messages from the PDE solver are printed on the current error message unit (see x04aaf).

$\mathbf{itrace}=1$

Output from the underlying ODE solver is printed on the current advisory message unit (see x04abf). This output contains details of Jacobian entries, the nonlinear iteration and the time integration during the computation of the ODE system.

$\mathbf{itrace}=2$

Output from the underlying ODE solver is similar to that produced when $\mathbf{itrace}=1$, except that the advisory messages are given in greater detail.

$\mathbf{itrace}\ge 3$

Output from the underlying ODE solver is similar to that produced when $\mathbf{itrace}=2$, except that the advisory messages are given in greater detail.

You are advised to set $\mathbf{itrace}=0$, unless you are experienced with Sub-chapter D02M–N.

17: $\mathbf{ind}$ – Integer Input/Output

On entry: indicates whether this is a continuation call or a new integration.

$\mathbf{ind}=0$

Starts or restarts the integration in time.

$\mathbf{ind}=1$

Continues the integration after an earlier exit from the routine. In this case, only the arguments tout and ifail should be reset between calls to d03pef.

Constraint: $\mathbf{ind}=0$ or $1$.

On exit: $\mathbf{ind}=1$.

18: $\mathbf{ifail}$ – Integer Input/Output

On entry: ifail must be set to $0$, $\mathrm{-1}$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.

A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $\mathrm{-1}$ means that an error message is printed while a value of $1$ means that it is not.

If halting is not appropriate, the value $\mathrm{-1}$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ or $\mathbf{1}$ is used it is essential to test the value of ifail on exit.

On exit: $\mathbf{ifail}=\mathbf{0}$ unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

$\mathbf{ifail}=1$

On entry, $\mathbf{acc}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{acc}>0.0$.

On entry, $\mathit{i}=⟨\mathit{\text{value}}⟩$, $\mathbf{x}\left(\mathit{i}\right)=⟨\mathit{\text{value}}⟩$, $\mathit{j}=⟨\mathit{\text{value}}⟩$ and $\mathbf{x}\left(\mathit{j}\right)=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{x}\left(1\right)<\mathbf{x}\left(2\right)<\cdots <\mathbf{x}\left(\mathbf{npts}\right)$.

On entry, $\mathbf{ind}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{ind}=0$ or $1$.

On entry, $\mathbf{itask}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{itask}=1$, $2$ or $3$.

On entry, $\mathbf{lisave}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{lisave}\ge ⟨\mathit{\text{value}}⟩$.

On entry, $\mathbf{lrsave}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{lrsave}\ge ⟨\mathit{\text{value}}⟩$.

On entry, $\mathbf{nleft}=⟨\mathit{\text{value}}⟩$, $\mathbf{npde}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{nleft}\le \mathbf{npde}$.

On entry, $\mathbf{nleft}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{nleft}\ge 0$.

On entry, $\mathbf{npde}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{npde}\ge 1$.

On entry, $\mathbf{npts}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{npts}\ge 3$.

On entry, on initial entry $\mathbf{ind}=1$.
Constraint: on initial entry $\mathbf{ind}=0$.

On entry, $\mathbf{tout}=⟨\mathit{\text{value}}⟩$ and $\mathbf{ts}=⟨\mathit{\text{value}}⟩$.
Constraint: $\mathbf{tout}>\mathbf{ts}$.

On entry, $\mathbf{tout}-\mathbf{ts}$ is too small: $\mathbf{tout}=⟨\mathit{\text{value}}⟩$ and $\mathbf{ts}=⟨\mathit{\text{value}}⟩$.

$\mathbf{ifail}=2$

Underlying ODE solver cannot make further progress from the point ts with the supplied value of acc. $\mathbf{ts}=⟨\mathit{\text{value}}⟩$, $\mathbf{acc}=⟨\mathit{\text{value}}⟩$.

$\mathbf{ifail}=3$

Repeated errors in an attempted step of underlying ODE solver. Integration was successful as far as ts: $\mathbf{ts}=⟨\mathit{\text{value}}⟩$.

In the underlying ODE solver, there were repeated errors or corrector convergence test failures on an attempted step, before completing the requested task. The problem may have a singularity or acc is too small for the integration to continue. Incorrect positioning of boundary conditions may also result in this error. Integration was successful as far as $t=\mathbf{ts}$.

$\mathbf{ifail}=4$

In setting up the ODE system an internal auxiliary was unable to initialize the derivative. This could be due to your setting $\mathbf{ires}=3$ in pdedef or bndary.

$\mathbf{ifail}=5$

Singular Jacobian of ODE system. Check problem formulation.

$\mathbf{ifail}=6$

In evaluating residual of ODE system, $\mathbf{ires}=2$ has been set in pdedef or bndary. Integration is successful as far as ts: $\mathbf{ts}=⟨\mathit{\text{value}}⟩$.

$\mathbf{ifail}=7$

acc was too small to start integration: $\mathbf{acc}=⟨\mathit{\text{value}}⟩$.

$\mathbf{ifail}=8$

ires set to an invalid value in call to pdedef or bndary.

$\mathbf{ifail}=9$

Serious error in internal call to an auxiliary. Increase itrace for further details.

$\mathbf{ifail}=10$

Integration completed, but a small change in acc is unlikely to result in a changed solution. $\mathbf{acc}=⟨\mathit{\text{value}}⟩$.

The required task has been completed, but it is estimated that a small change in acc is unlikely to produce any change in the computed solution. (Only applies when you are not operating in one step mode, that is when $\mathbf{itask}\ne 2$.)

$\mathbf{ifail}=11$

Error during Jacobian formulation for ODE system. Increase itrace for further details.

$\mathbf{ifail}=-99$

An unexpected error has been triggered by this routine. Please contact NAG.

See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$\mathbf{ifail}=-399$

Your licence key may have expired or may not have been installed correctly.

See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$\mathbf{ifail}=-999$

Dynamic memory allocation failed.

See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results