NAG Library Routine Document
d03pef (dim1_parab_keller)
1
Purpose
d03pef integrates a system of linear or nonlinear, firstorder, timedependent partial differential equations (PDEs) in one space variable. The spatial discretization is performed using the Keller box scheme and the method of lines is employed to reduce the PDEs to a system of ordinary differential equations (ODEs). The resulting system is solved using a Backward Differentiation Formula (BDF) method.
2
Specification
Fortran Interface
Subroutine d03pef ( 
npde, ts, tout, pdedef, bndary, u, npts, x, nleft, acc, rsave, lrsave, isave, lisave, itask, itrace, ind, ifail) 
Integer, Intent (In)  ::  npde, npts, nleft, lrsave, lisave, itask, itrace  Integer, Intent (Inout)  ::  isave(lisave), ind, ifail  Real (Kind=nag_wp), Intent (In)  ::  tout, x(npts), acc  Real (Kind=nag_wp), Intent (Inout)  ::  ts, u(npde,npts), rsave(lrsave)  External  ::  pdedef, bndary 

C Header Interface
#include nagmk26.h
void 
d03pef_ (const Integer *npde, double *ts, const double *tout, void (NAG_CALL *pdedef)(const Integer *npde, const double *t, const double *x, const double u[], const double ut[], const double ux[], double res[], Integer *ires), void (NAG_CALL *bndary)(const Integer *npde, const double *t, const Integer *ibnd, const Integer *nobc, const double u[], const double ut[], double res[], Integer *ires), double u[], const Integer *npts, const double x[], const Integer *nleft, const double *acc, double rsave[], const Integer *lrsave, Integer isave[], const Integer *lisave, const Integer *itask, const Integer *itrace, Integer *ind, Integer *ifail) 

3
Description
d03pef integrates the system of firstorder PDEs
In particular the functions
${G}_{i}$ must have the general form
where
${P}_{i,j}$ and
${Q}_{i}$ depend on
$x$,
$t$,
$U$,
${U}_{x}$ and the vector
$U$ is the set of solution values
and the vector
${U}_{x}$ is its partial derivative with respect to
$x$. Note that
${P}_{i,j}$ and
${Q}_{i}$ must not depend on
$\frac{\partial U}{\partial t}$.
The integration in time is from ${t}_{0}$ to ${t}_{\mathrm{out}}$, over the space interval $a\le x\le b$, where $a={x}_{1}$ and $b={x}_{{\mathbf{npts}}}$ are the leftmost and rightmost points of a userdefined mesh ${x}_{1},{x}_{2},\dots ,{x}_{{\mathbf{npts}}}$. The mesh should be chosen in accordance with the expected behaviour of the solution.
The PDE system which is defined by the functions
${G}_{i}$ must be specified in
pdedef.
The initial values of the functions
$U\left(x,t\right)$ must be given at
$t={t}_{0}$. For a firstorder system of PDEs, only one boundary condition is required for each PDE component
${U}_{i}$. The
npde boundary conditions are separated into
${n}_{a}$ at the lefthand boundary
$x=a$, and
${n}_{b}$ at the righthand boundary
$x=b$, such that
${n}_{a}+{n}_{b}={\mathbf{npde}}$. The position of the boundary condition for each component should be chosen with care; the general rule is that if the characteristic direction of
${U}_{i}$ at the lefthand boundary (say) points into the interior of the solution domain, then the boundary condition for
${U}_{i}$ should be specified at the lefthand boundary. Incorrect positioning of boundary conditions generally results in initialization or integration difficulties in the underlying time integration routines.
The boundary conditions have the form:
at the lefthand boundary, and
at the righthand boundary.
Note that the functions
${G}_{i}^{L}$ and
${G}_{i}^{R}$ must not depend on
${U}_{x}$, since spatial derivatives are not determined explicitly in the Keller box scheme (see
Keller (1970)). If the problem involves derivative (Neumann) boundary conditions then it is generally possible to restate such boundary conditions in terms of permissible variables. Also note that
${G}_{i}^{L}$ and
${G}_{i}^{R}$ must be linear with respect to time derivatives, so that the boundary conditions have the general form
at the lefthand boundary, and
at the righthand boundary, where
${E}_{i,j}^{L}$,
${E}_{i,j}^{R}$,
${S}_{i}^{L}$, and
${S}_{i}^{R}$ depend on
$x$,
$t$ and
$U$ only.
The boundary conditions must be specified in
bndary.
The problem is subject to the following restrictions:
(i) 
${t}_{0}<{t}_{\mathrm{out}}$, so that integration is in the forward direction; 
(ii) 
${P}_{i,j}$ and ${Q}_{i}$ must not depend on any time derivatives; 
(iii) 
The evaluation of the function ${G}_{i}$ is done at the midpoints of the mesh intervals by calling the pdedef for each midpoint in turn. Any discontinuities in the function must therefore be at one or more of the mesh points ${x}_{1},{x}_{2},\dots ,{x}_{{\mathbf{npts}}}$; 
(iv) 
At least one of the functions ${P}_{i,j}$ must be nonzero so that there is a time derivative present in the problem. 
In this method of lines approach the Keller box scheme (see
Keller (1970)) is applied to each PDE in the space variable only, resulting in a system of ODEs in time for the values of
${U}_{i}$ at each mesh point. In total there are
${\mathbf{npde}}\times {\mathbf{npts}}$ ODEs in the time direction. This system is then integrated forwards in time using a BDF method.
4
References
Berzins M (1990) Developments in the NAG Library software for parabolic equations Scientific Software Systems (eds J C Mason and M G Cox) 59–72 Chapman and Hall
Berzins M, Dew P M and Furzeland R M (1989) Developing software for timedependent problems using the method of lines and differentialalgebraic integrators Appl. Numer. Math. 5 375–397
Keller H B (1970) A new difference scheme for parabolic problems Numerical Solutions of Partial Differential Equations (ed J Bramble) 2 327–350 Academic Press
Pennington S V and Berzins M (1994) New NAG Library software for firstorder partial differential equations ACM Trans. Math. Softw. 20 63–99
5
Arguments
 1: $\mathbf{npde}$ – IntegerInput

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
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
#include nagmk26.h
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: $\mathbf{npde}$ – IntegerInput

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) arrayInput

On entry: ${\mathbf{u}}\left(\mathit{i}\right)$ contains the value of the component ${U}_{\mathit{i}}\left(x,t\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
 5: $\mathbf{ut}\left({\mathbf{npde}}\right)$ – Real (Kind=nag_wp) arrayInput

On entry: ${\mathbf{ut}}\left(\mathit{i}\right)$ contains the value of the component $\frac{\partial {U}_{\mathit{i}}\left(x,t\right)}{\partial t}$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
 6: $\mathbf{ux}\left({\mathbf{npde}}\right)$ – Real (Kind=nag_wp) arrayInput

On entry: ${\mathbf{ux}}\left(\mathit{i}\right)$ contains the value of the component $\frac{\partial {U}_{\mathit{i}}\left(x,t\right)}{\partial x}$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
 7: $\mathbf{res}\left({\mathbf{npde}}\right)$ – Real (Kind=nag_wp) arrayOutput

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
i.e., only terms depending explicitly on time derivatives, or
i.e., all terms in equation
(2).
The definition of
$G$ is determined by the input value of
ires.
 8: $\mathbf{ires}$ – IntegerInput/Output

On entry: the form of
${G}_{i}$ that must be returned in the array
res.
 ${\mathbf{ires}}=1$
 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 floatingpoint 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
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
#include nagmk26.h
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: $\mathbf{npde}$ – IntegerInput

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: $\mathbf{ibnd}$ – IntegerInput

On entry: determines the position of the boundary conditions.
 ${\mathbf{ibnd}}=0$
 bndary must compute the lefthand boundary condition at $x=a$.
 ${\mathbf{ibnd}}\ne 0$
 Indicates that bndary must compute the righthand boundary condition at $x=b$.
 4: $\mathbf{nobc}$ – IntegerInput

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) arrayInput

On entry:
${\mathbf{u}}\left(\mathit{i}\right)$ contains the value of the component
${U}_{\mathit{i}}\left(x,t\right)$ 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) arrayInput

On entry:
${\mathbf{ut}}\left(\mathit{i}\right)$ contains the value of the component
$\frac{\partial {U}_{\mathit{i}}\left(x,t\right)}{\partial t}$ 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) arrayOutput

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
i.e., only terms depending explicitly on time derivatives, or
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: $\mathbf{ires}$ – IntegerInput/Output

On entry: the form
${G}_{i}^{L}$ (or
${G}_{i}^{R}$) that must be returned in the array
res.
 ${\mathbf{ires}}=1$
 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 floatingpoint 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: $\mathbf{u}\left({\mathbf{npde}},{\mathbf{npts}}\right)$ – Real (Kind=nag_wp) arrayInput/Output

On entry: the initial values of $U\left(x,t\right)$ at $t={\mathbf{ts}}$ and the mesh points
${\mathbf{x}}\left(\mathit{j}\right)$, for $\mathit{j}=1,2,\dots ,{\mathbf{npts}}$.
On exit: ${\mathbf{u}}\left(\mathit{i},\mathit{j}\right)$ will contain the computed solution at $t={\mathbf{ts}}$.
 7: $\mathbf{npts}$ – IntegerInput

On entry: the number of mesh points in the interval $\left[a,b\right]$.
Constraint:
${\mathbf{npts}}\ge 3$.
 8: $\mathbf{x}\left({\mathbf{npts}}\right)$ – Real (Kind=nag_wp) arrayInput

On entry: the mesh points in the spatial direction. ${\mathbf{x}}\left(1\right)$ must specify the lefthand boundary, $a$, and ${\mathbf{x}}\left({\mathbf{npts}}\right)$ must specify the righthand boundary, $b$.
Constraint:
${\mathbf{x}}\left(1\right)<{\mathbf{x}}\left(2\right)<\cdots <{\mathbf{x}}\left({\mathbf{npts}}\right)$.
 9: $\mathbf{nleft}$ – IntegerInput

On entry: the number ${n}_{a}$ of boundary conditions at the lefthand 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
$E\left(i,j\right)$ is the estimated error for
${U}_{i}$ at the
$j$th mesh point, the error test is:
Constraint:
${\mathbf{acc}}>0.0$.
 11: $\mathbf{rsave}\left({\mathbf{lrsave}}\right)$ – Real (Kind=nag_wp) arrayCommunication 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: $\mathbf{lrsave}$ – IntegerInput

On entry: the dimension of the array
rsave as declared in the (sub)program from which
d03pef is called.
Constraint:
${\mathbf{lrsave}}\ge \left(4\times {\mathbf{npde}}+{\mathbf{nleft}}+14\right)\times {\mathbf{npde}}\times {\mathbf{npts}}+\left(3\times {\mathbf{npde}}+21\right)\times {\mathbf{npde}}+\phantom{\rule{0ex}{0ex}}7\times {\mathbf{npts}}+54$.
 13: $\mathbf{isave}\left({\mathbf{lisave}}\right)$ – Integer arrayCommunication 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 backsubstitution using the $LU$ decomposition of the Jacobian matrix.
 14: $\mathbf{lisave}$ – IntegerInput

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: $\mathbf{itask}$ – IntegerInput

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: $\mathbf{itrace}$ – IntegerInput

On entry: the level of trace information required from
d03pef and the underlying ODE solver as follows:
 ${\mathbf{itrace}}\le 1$
 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
Subchapter D02M–N.
 17: $\mathbf{ind}$ – IntegerInput/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: $\mathbf{ifail}$ – IntegerInput/Output

On entry:
ifail must be set to
$0$,
$1\text{ or}1$. If you are unfamiliar with this argument you should refer to
Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
$1\text{ or}1$ is recommended. If the output of error messages is undesirable, then the value
$1$ is recommended. Otherwise, if you are not familiar with this argument, the recommended value is
$0$.
When the value $\mathbf{1}\text{ or}\mathbf{1}$ 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
If on entry
${\mathbf{ifail}}=0$ or
$1$, explanatory error messages are output on the current error message unit (as defined by
x04aaf).
Errors or warnings detected by the routine:
 ${\mathbf{ifail}}=1$

On entry,  ${\mathbf{tout}}\le {\mathbf{ts}}$, 
or  $\left({\mathbf{tout}}{\mathbf{ts}}\right)$ is too small, 
or  ${\mathbf{itask}}\ne 1$, $2$ or $3$, 
or  $\text{mesh points}{\mathbf{x}}\left(i\right)$ are not ordered correctly, 
or  ${\mathbf{npts}}<3$, 
or  ${\mathbf{npde}}<1$, 
or  nleft is not in the range $0$ to npde, 
or  ${\mathbf{acc}}\le 0.0$, 
or  ${\mathbf{ind}}\ne 0$ or $1$, 
or  lrsave is too small, 
or  lisave is too small, 
or  d03pef called initially with ${\mathbf{ind}}=1$. 
 ${\mathbf{ifail}}=2$

The underlying ODE solver cannot make any further progress across the integration range from the current point
$t={\mathbf{ts}}$ with the supplied value of
acc. The components of
u contain the computed values at the current point
$t={\mathbf{ts}}$.
 ${\mathbf{ifail}}=3$

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, the internal initialization routine was unable to initialize the derivative of the ODE system. This could be due to the fact that
ires was repeatedly set to
$3$ in the
pdedef or
bndary, when the residual in the underlying ODE solver was being evaluated. Incorrect positioning of boundary conditions may also result in this error.
 ${\mathbf{ifail}}=5$

In solving the ODE system, a singular Jacobian has been encountered. You should check their problem formulation.
 ${\mathbf{ifail}}=6$

When evaluating the residual in solving the ODE system,
ires was set to
$2$ in one of
pdedef or
bndary. Integration was successful as far as
$t={\mathbf{ts}}$.
 ${\mathbf{ifail}}=7$

The value of
acc is so small that the routine is unable to start the integration in time.
 ${\mathbf{ifail}}=8$

In either,
pdedef or
bndary,
ires was set to an invalid value.
 ${\mathbf{ifail}}=9$ (d02nnf)

A serious error has occurred in an internal call to the specified routine. Check the problem specification and all arguments and array dimensions. Setting
${\mathbf{itrace}}=1$ may provide more information. If the problem persists, contact
NAG.
 ${\mathbf{ifail}}=10$

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
${\mathbf{itask}}\ne 2$.)
 ${\mathbf{ifail}}=11$

An error occurred during Jacobian formulation of the ODE system (a more detailed error description may be directed to the current advisory message unit).
 ${\mathbf{ifail}}=99$
An unexpected error has been triggered by this routine. Please
contact
NAG.
See
Section 3.9 in How to Use the NAG Library and its Documentation for further information.
 ${\mathbf{ifail}}=399$
Your licence key may have expired or may not have been installed correctly.
See
Section 3.8 in How to Use the NAG Library and its Documentation for further information.
 ${\mathbf{ifail}}=999$
Dynamic memory allocation failed.
See
Section 3.7 in How to Use the NAG Library and its Documentation for further information.
7
Accuracy
d03pef controls the accuracy of the integration in the time direction but not the accuracy of the approximation in space. The spatial accuracy depends on both the number of mesh points and on their distribution in space. In the time integration only the local error over a single step is controlled and so the accuracy over a number of steps cannot be guaranteed. You should therefore test the effect of varying the accuracy argument,
acc.
8
Parallelism and Performance
d03pef is not thread safe and should not be called from a multithreaded user program. Please see
Section 3.12.1 in How to Use the NAG Library and its Documentation for more information on thread safety.
d03pef is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
d03pef 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
X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the
Users' Note for your implementation for any additional implementationspecific information.
The Keller box scheme can be used to solve higherorder problems which have been reduced to firstorder by the introduction of new variables (see the example problem in
d03pkf). In general, a secondorder problem can be solved with slightly greater accuracy using the Keller box scheme instead of a finite difference scheme (
d03pcf/d03pca or
d03phf/d03pha for example), but at the expense of increased CPU time due to the larger number of function evaluations required.
It should be noted that the Keller box scheme, in common with other centraldifference schemes, may be unsuitable for some hyperbolic firstorder problems such as the apparently simple linear advection equation
${U}_{t}+a{U}_{x}=0$, where
$a$ is a constant, resulting in spurious oscillations due to the lack of dissipation. This type of problem requires a discretization scheme with upwind weighting (
d03pff for example), or the addition of a secondorder artificial dissipation term.
The time taken depends on the complexity of the system and on the accuracy requested.
10
Example
This example is the simple firstorder system
for
$t\in \left[0,1\right]$ and
$x\in \left[0,1\right]$.
The initial conditions are
and the Dirichlet boundary conditions for
${U}_{1}$ at
$x=0$ and
${U}_{2}$ at
$x=1$ are given by the exact solution:
10.1
Program Text
Program Text (d03pefe.f90)
10.2
Program Data
Program Data (d03pefe.d)
10.3
Program Results
Program Results (d03pefe.r)