NAG FL Interface
d02ngf (ivp_​stiff_​imp_​fulljac)

1 Purpose

d02ngf is a direct communication routine for integrating stiff systems of implicit ordinary differential equations coupled with algebraic equations when the Jacobian is a full matrix.

2 Specification

Fortran Interface
Subroutine d02ngf ( neq, ldysav, t, tout, y, ydot, rwork, rtol, atol, itol, inform, resid, ysav, sdysav, jac, wkjac, nwkjac, monitr, lderiv, itask, itrace, ifail)
Integer, Intent (In) :: neq, ldysav, itol, sdysav, nwkjac, itask, itrace
Integer, Intent (Inout) :: inform(23), ifail
Real (Kind=nag_wp), Intent (In) :: rtol(*), atol(*)
Real (Kind=nag_wp), Intent (Inout) :: t, tout, y(neq), ydot(neq), rwork(50+4*neq), ysav(ldysav,sdysav), wkjac(nwkjac)
Logical, Intent (Inout) :: lderiv(2)
External :: resid, jac, monitr
C Header Interface
#include <nag.h>
void  d02ngf_ (const Integer *neq, const Integer *ldysav, double *t, double *tout, double y[], double ydot[], double rwork[], const double rtol[], const double atol[], const Integer *itol, Integer inform[],
void (NAG_CALL *resid)(const Integer *neq, const double *t, const double y[], const double ydot[], double r[], Integer *ires),
double ysav[], const Integer *sdysav,
void (NAG_CALL *jac)(const Integer *neq, const double *t, const double y[], const double ydot[], const double *h, const double *d, double p[]),
double wkjac[], const Integer *nwkjac,
void (NAG_CALL *monitr)(const Integer *neq, const Integer *ldysav, const double *t, const double *hlast, double *hnext, double y[], const double ydot[], const double ysav[], const double r[], const double acor[], Integer *imon, Integer *inln, double *hmin, double *hmax, const Integer *nqu),
logical lderiv[], const Integer *itask, const Integer *itrace, Integer *ifail)
The routine may be called by the names d02ngf or nagf_ode_ivp_stiff_imp_fulljac.

3 Description

d02ngf is a general purpose routine for integrating the initial value problem for a stiff system of implicit ordinary differential equations coupled with algebraic equations, written in the form
At,yy=gt,y.  
It is designed specifically for the case where the resulting Jacobian is a full matrix (see the description of jac).
Both interval and step oriented modes of operation are available and also modes designed to permit intermediate output within an interval oriented mode.
An outline of a typical calling program for d02ngf is given below. It calls the full matrix linear algebra setup routine d02nsf, the Backward Differentiation Formula (BDF) integrator setup routine d02nvf, and its diagnostic counterpart d02nyf.
!     Declarations

      External resid, jac, monitr
          .
          .
          .
      ifail = 0
      Call d02nvf(...,ifail)
      Call d02nsf(neq, neqmax, jceval, nwkjac, rwork, ifail)
      ifail = -1
      Call d02ngf(neq, neqmax, t, tout, y, ydot, rwork, rtol, &
                  atol, itol, inform, resid, ysave, ny2dim,   &
     		  jac, wkjac, nwkjac, monitr, lderiv, itask,  &		  itrace, ifail)
      If (ifail.eq.1 .or. ifail.ge.14) Stop
      ifail = 0
      Call d02nyf(...)
          .
          .
          .
      Stop
      End
The linear algebra setup routine d02nsf and one of the integrator setup routines, d02mvf, d02nvf or d02nwf, must be called prior to the call of d02ngf. The integrator diagnostic routine d02nyf may be called after the call to d02ngf. There is also a routine, d02nzf, designed to permit you to change step size on a continuation call to d02ngf without restarting the integration process.

4 References

See the D02MN Sub-chapter Introduction.

5 Arguments

1: neq Integer Input
On entry: the number of differential equations to be solved.
Constraint: neq1.
2: ldysav Integer Input
On entry: a bound on the maximum number of equations to be solved during the integration.
Constraint: ldysavneq.
3: t Real (Kind=nag_wp) Input/Output
On entry: t, the value of the independent variable. The input value of t is used only on the first call as the initial point of the integration.
On exit: the value at which the computed solution y is returned (usually at tout).
4: tout Real (Kind=nag_wp) Input/Output
On entry: the next value of t at which a computed solution is desired. For the initial t, the input value of tout is used to determine the direction of integration. Integration is permitted in either direction (see also itask).
Constraint: toutt.
On exit: normally unchanged. However, when itask=6, tout contains the value of t at which initial values have been computed without performing any integration. See descriptions of itask and lderiv.
5: yneq Real (Kind=nag_wp) array Input/Output
On entry: the values of the dependent variables (solution). On the first call the first neq elements of y must contain the vector of initial values.
On exit: the computed solution vector, evaluated at t (usually t=tout).
6: ydotneq Real (Kind=nag_wp) array Input/Output
On entry: if lderiv1=.TRUE., ydot must contain approximations to the time derivatives y of the vector y.
If lderiv1=.FALSE., ydot need not be set on entry.
On exit: the time derivatives y of the vector y at the last integration point.
7: rwork50+4×neq Real (Kind=nag_wp) array Communication Array
8: rtol* Real (Kind=nag_wp) array Input
Note: the dimension of the array rtol must be at least 1 if itol=1 or 2, and at least neq otherwise.
On entry: the relative local error tolerance.
Constraint: rtoli0.0 for all relevant i (see itol).
9: atol* Real (Kind=nag_wp) array Input
Note: the dimension of the array atol must be at least 1 if itol=1 or 3, and at least neq otherwise.
On entry: the absolute local error tolerance.
Constraint: atoli0.0 for all relevant i (see itol).
10: itol Integer Input
On entry: a value to indicate the form of the local error test. itol indicates to d02ngf whether to interpret either or both of rtol or atol as a vector or a scalar. The error test to be satisfied is ei/wi<1.0, where wi is defined as follows:
itol rtol atol wi
1 scalar scalar rtol1×yi+atol1
2 scalar vector rtol1×yi+atoli
3 vector scalar rtoli×yi+atol1
4 vector vector rtoli×yi+atoli
ei is an estimate of the local error in yi, computed internally, and the choice of norm to be used is defined by a previous call to an integrator setup routine.
Constraint: itol=1, 2, 3 or 4.
11: inform23 Integer array Communication Array
12: resid Subroutine, supplied by the user. External Procedure
resid must evaluate the residual
r=gt,y-At,yy  
in one case and
r=-At,yy  
in another.
The specification of resid is:
Fortran Interface
Subroutine resid ( neq, t, y, ydot, r, ires)
Integer, Intent (In) :: neq
Integer, Intent (Inout) :: ires
Real (Kind=nag_wp), Intent (In) :: t, y(neq), ydot(neq)
Real (Kind=nag_wp), Intent (Out) :: r(neq)
C Header Interface
void  resid_ (const Integer *neq, const double *t, const double y[], const double ydot[], double r[], Integer *ires)
1: neq Integer Input
On entry: the number of equations being solved.
2: t Real (Kind=nag_wp) Input
On entry: t, the current value of the independent variable.
3: yneq Real (Kind=nag_wp) array Input
On entry: the value of yi, for i=1,2,,neq.
4: ydotneq Real (Kind=nag_wp) array Input
On entry: the value of yi, for i=1,2,,neq, at t.
5: rneq Real (Kind=nag_wp) array Output
On exit: ri must contain the ith component of r, for i=1,2,,neq, where
r=gt,y-At,yy (1)
or
r=-At,yy (2)
and where the definition of r is determined by the input value of ires.
6: ires Integer Input/Output
On entry: the form of the residual that must be returned in array r.
ires=-1
The residual defined in equation (2) must be returned.
ires=1
The residual defined in equation (1) must be returned.
On exit: should be unchanged unless one of the following actions is required of the integrator, in which case ires should be set accordingly.
ires=2
Indicates to the integrator that control should be passed back immediately to the calling (sub)program with the error indicator set to ifail=11.
ires=3
Indicates to the integrator that an error condition has occurred in the solution vector, its time derivative or in the value of t. The integrator will use a smaller time step to try to avoid this condition. If this is not possible, the integrator returns to the calling (sub)program with the error indicator set to ifail=7.
ires=4
Indicates to the integrator to stop its current operation and to enter monitr immediately with argument imon=-2.
resid must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which d02ngf is called. Arguments denoted as Input must not be changed by this procedure.
Note: resid should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by d02ngf. If your code inadvertently does return any NaNs or infinities, d02ngf is likely to produce unexpected results.
13: ysavldysavsdysav Real (Kind=nag_wp) array Communication Array
14: sdysav Integer Input
On entry: the second dimension of the array ysav as declared in the (sub)program from which d02ngf is called. An appropriate value for sdysav is described in the specifications of the integrator setup routines d02mvf, d02nvf and d02nwf. This value must be the same as that supplied to the integrator setup routine.
15: jac Subroutine, supplied by the NAG Library or the user. External Procedure
jac must evaluate the Jacobian of the system. If this option is not required, the actual argument for jac must be the dummy routine d02ngz. (d02ngz is included in the NAG Library.) You must indicate to the integrator whether this option is to be used by setting the argument jceval appropriately in a call to the full linear algebra setup routine d02nsf.
First we must define the system of nonlinear equations which is solved internally by the integrator. The time derivative, y, generated internally, has the form
y = y-z / hd ,  
where h is the current step size and d is an argument that depends on the integration method in use. The vector y is the current solution and the vector z depends on information from previous time steps. This means that d dy ​ ​ = hd d dy ​ ​ . The system of nonlinear equations that is solved has the form
A t,y y - g t,y = 0  
but it is solved in the form
r t,y = 0 ,  
where r is the function defined by
r t,y = hd A t,y y-z / hd - g t,y .  
It is the Jacobian matrix r y that you must supply in jac as follows:
ri yj = aij t,y + hd yj k=1 neq aik t,y yk - gi t,y .  
The specification of jac is:
Fortran Interface
Subroutine jac ( neq, t, y, ydot, h, d, p)
Integer, Intent (In) :: neq
Real (Kind=nag_wp), Intent (In) :: t, y(neq), ydot(neq), h, d
Real (Kind=nag_wp), Intent (Inout) :: p(neq,neq)
C Header Interface
void  jac_ (const Integer *neq, const double *t, const double y[], const double ydot[], const double *h, const double *d, double p[])
1: neq Integer Input
On entry: the number of equations being solved.
2: t Real (Kind=nag_wp) Input
On entry: t, the current value of the independent variable.
3: yneq Real (Kind=nag_wp) array Input
On entry: yi, for i=1,2,,neq, the current solution component.
4: ydotneq Real (Kind=nag_wp) array Input
On entry: the derivative of the solution at the current point t.
5: h Real (Kind=nag_wp) Input
On entry: the current step size.
6: d Real (Kind=nag_wp) Input
On entry: the argument d which depends on the integration method.
7: pneqneq Real (Kind=nag_wp) array Input/Output
On entry: is set to zero.
On exit: pij must contain ri yj , for i=1,2,,neq and j=1,2,,neq.
Only the nonzero elements of this array need be set, since it is preset to zero before the call to jac.
jac must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which d02ngf is called. Arguments denoted as Input must not be changed by this procedure.
Note: jac should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by d02ngf. If your code inadvertently does return any NaNs or infinities, d02ngf is likely to produce unexpected results.
16: wkjacnwkjac Real (Kind=nag_wp) array Communication Array
17: nwkjac Integer Input
On entry: the dimension of the array wkjac as declared in the (sub)program from which d02ngf is called. This value must be the same as that supplied to the linear algebra setup routine d02nsf.
Constraint: nwkjacldysav×ldysav+1.
18: monitr Subroutine, supplied by the NAG Library or the user. External Procedure
monitr performs tasks requested by you. If this option is not required, the actual argument for monitr must be the dummy routine d02nby. (d02nby is included in the NAG Library.)
The specification of monitr is:
Fortran Interface
Subroutine monitr ( neq, ldysav, t, hlast, hnext, y, ydot, ysav, r, acor, imon, inln, hmin, hmax, nqu)
Integer, Intent (In) :: neq, ldysav, nqu
Integer, Intent (Inout) :: imon
Integer, Intent (Out) :: inln
Real (Kind=nag_wp), Intent (In) :: t, hlast, ydot(neq), ysav(ldysav,*), r(neq), acor(neq,2)
Real (Kind=nag_wp), Intent (Inout) :: hnext, y(neq), hmin, hmax
C Header Interface
void  monitr_ (const Integer *neq, const Integer *ldysav, const double *t, const double *hlast, double *hnext, double y[], const double ydot[], const double ysav[], const double r[], const double acor[], Integer *imon, Integer *inln, double *hmin, double *hmax, const Integer *nqu)
1: neq Integer Input
On entry: the number of equations being solved.
2: ldysav Integer Input
On entry: an upper bound on the number of equations to be solved.
3: t Real (Kind=nag_wp) Input
On entry: the current value of the independent variable.
4: hlast Real (Kind=nag_wp) Input
On entry: the last step size successfully used by the integrator.
5: hnext Real (Kind=nag_wp) Input/Output
On entry: the step size that the integrator proposes to take on the next step.
On exit: the next step size to be used. If this is different from the input value, imon must be set to 4.
6: yneq Real (Kind=nag_wp) array Input/Output
On entry: y, the values of the dependent variables evaluated at t.
On exit: these values must not be changed unless imon is set to 2.
7: ydotneq Real (Kind=nag_wp) array Input
On entry: the time derivatives y of the vector y.
8: ysavldysav* Real (Kind=nag_wp) array Communication Array
Note: the second dimension of ysav is sdysav as in the call of d02ngf.
On entry: workspace to enable you to carry out interpolation using either of the routines d02xjf or d02xkf.
9: rneq Real (Kind=nag_wp) array Input
On entry: if imon=0 and inln=3, the first neq elements contain the residual vector At,yy-gt,y.
10: acorneq2 Real (Kind=nag_wp) array Input
On entry: with imon=1, acori1 contains the weight used for the ith equation when the norm is evaluated, and acori2 contains the estimated local error for the ith equation. The scaled local error at the end of a timestep may be obtained by calling the real function d02zaf as follows:
ifail = 1
errloc = d02zaf(neq, acor(1,2), acor(1,1), ifail)
! CHECK IFAIL BEFORE PROCEEDING
11: imon Integer Input/Output
On entry: a flag indicating under what circumstances monitr was called:
imon=-2
Entry from the integrator after ires=4 (set in resid) caused an early termination (this facility could be used to locate discontinuities).
imon=-1
The current step failed repeatedly.
imon=0
Entry after a call to the internal nonlinear equation solver (see inln).
imon=1
The current step was successful.
On exit: may be reset to determine subsequent action in d02ngf.
imon=-2
Integration is to be halted. A return will be made from the integrator to the calling (sub)program with ifail=12.
imon=-1
Allow the integrator to continue with its own internal strategy. The integrator will try up to three restarts unless imon-1 on exit.
imon=0
Return to the internal nonlinear equation solver, where the action taken is determined by the value of inln (see inln).
imon=1
Normal exit to the integrator to continue integration.
imon=2
Restart the integration at the current time point. The integrator will restart from order 1 when this option is used. The solution y, provided by monitr, will be used for the initial conditions.
imon=3
Try to continue with the same step size and order as was to be used before the call to monitr. hmin and hmax may be altered if desired.
imon=4
Continue the integration but using a new value of hnext and possibly new values of hmin and hmax.
12: inln Integer Output
On exit: the action to be taken by the internal nonlinear equation solver when monitr is exited with imon=0. By setting inln=3 and returning to the integrator, the residual vector is evaluated and placed in the array r, and then monitr is called again. At present this is the only option available: inln must not be set to any other value.
13: hmin Real (Kind=nag_wp) Input/Output
On entry: the minimum step size to be taken on the next step.
On exit: the minimum step size to be used. If this is different from the input value, imon must be set to 3 or 4.
14: hmax Real (Kind=nag_wp) Input/Output
On entry: the maximum step size to be taken on the next step.
On exit: the maximum step size to be used. If this is different from the input value, imon must be set to 3 or 4. If hmax is set to zero, no limit is assumed.
15: nqu Integer Input
On entry: the order of the integrator used on the last step. This is supplied to enable you to carry out interpolation using either of the routines d02xjf or d02xkf.
monitr must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which d02ngf is called. Arguments denoted as Input must not be changed by this procedure.
Note: monitr should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by d02ngf. If your code inadvertently does return any NaNs or infinities, d02ngf is likely to produce unexpected results.
19: lderiv2 Logical array Input/Output
On entry: lderiv1 must be set to .TRUE. if you have supplied both an initial y and an initial y. lderiv1 must be set to .FALSE. if only the initial y has been supplied.
lderiv2 must be set to .TRUE. if the integrator is to use a modified Newton method to evaluate the initial y and y. Note that y and y, if supplied, are used as initial estimates. This method involves taking a small step at the start of the integration, and if itask=6 on entry, t and tout will be set to the result of taking this small step. lderiv2 must be set to .FALSE. if the integrator is to use functional iteration to evaluate the initial y and y, and if this fails a modified Newton method will then be attempted. lderiv2=.TRUE. is recommended if there are implicit equations or the initial y and y are zero.
On exit: lderiv1 is normally unchanged. However if itask=6 and internal initialization was successful then lderiv1=.TRUE..
lderiv2=.TRUE., if implicit equations were detected. Otherwise lderiv2=.FALSE..
20: itask Integer Input
On entry: the task to be performed by the integrator.
itask=1
Normal computation of output values of yt at t=tout (by overshooting and interpolating).
itask=2
Take one step only and return.
itask=3
Stop at the first internal integration point at or beyond t=tout and return.
itask=4
Normal computation of output values of yt at t=tout but without overshooting t=tcrit. tcrit must be specified as an option in one of the integrator setup routines before the first call to the integrator, or specified in the optional input routine before a continuation call. tcrit may be equal to or beyond tout, but not before it, in the direction of integration.
itask=5
Take one step only and return, without passing tcrit. tcrit must be specified as under itask=4.
itask=6
The integrator will solve for the initial values of y and y only and then return to the calling (sub)program without doing the integration. This option can be used to check the initial values of y and y. Functional iteration or a ‘small’ backward Euler method used in conjunction with a damped Newton iteration is used to calculate these values (see lderiv). Note that if a backward Euler step is used then the value of t will have been advanced a short distance from the initial point.
Note:  if d02ngf is recalled with a different value of itask (and tout altered), the initialization procedure is repeated, possibly leading to different initial conditions.
Constraint: 1itask6.
21: itrace Integer Input
On entry: the level of output that is printed by the integrator. itrace may take the value -1, 0, 1, 2 or 3.
itrace<-1
-1 is assumed and similarly if itrace>3, 3 is assumed.
itrace=-1
No output is generated.
itrace=0
Only warning messages are printed on the current error message unit (see x04aaf).
itrace>0
Warning messages are printed as above, and on the current advisory message unit (see x04abf) output is generated which details Jacobian entries, the nonlinear iteration and the time integration. The advisory messages are given in greater detail the larger the value of itrace.
22: ifail Integer Input/Output
On entry: ifail must be set to 0, -1 or 1. If you are unfamiliar with this argument you should refer to Section 4 in the Introduction to the NAG Library FL Interface for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1 or 1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, because for this routine the values of the output arguments may be useful even if ifail0 on exit, the recommended value is -1. When the value -1 or 1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry 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:
ifail=1
Either the integrator setup routine has not been called prior to the first call of this routine, or a communication array has become corrupted.
Either the linear algebra setup routine has not been called prior to the first call of this routine, or a communication array has become corrupted.
Either the routine was entered on a continuation call without a prior call of this routine, or a communication array has become corrupted.
Either the value of nwkjac is not the same as the value supplied to the setup routine or a communication array has become corrupted.
nwkjac=value and nwkjac=value in d02nsf.
Either the value of sdysav is not the same as the value supplied to the setup routine or a communication array has become corrupted.
sdysav=value, sdysav (setup) =value.
Failure during internal time interpolation. tout and the current time are too close.
itask=value and tout=value and the current time is value.
ires was set to an illegal value during initialization.
ires=value at time value.
itask=3 and tout is more than an integration step behind the current time.
tout=value, current time minus step size: value.
monitr appears to have overwritten the solution vector.
Further integration will not be attempted.
monitr set imon=value.
Constraint: -2imon4.
monitr set inln=value.
Constraint: inln=3.
On entry, an illegal (negative) maximum number of steps was provided in a prior call to a setup routine. maxstp=value.
On entry, an illegal (negative) maximum stepsize was provided in a prior call to a setup routine. hmax=value.
On entry, an illegal (negative) minimum stepsize was provided in a prior call to a setup routine. hmin=value.
On entry, atol=value.
Constraint: atol0.0.
On entry, ires=value and dydt=0.0 for all elements.
Check the evaluation of the residual for this value of ires.
On entry, itask=value.
Constraint: 1itask5.
On entry, itask=4 or 5 and tcrit is before the current time in the direction of integration.
itask=value, tcrit=value and the current time is value.
On entry, itask=4 or 5 and tcrit is before tout in the direction of integration.
itask=value, tcrit=value and tout=value.
On entry, itol=value.
Constraint: 1itol4.
On entry, neq=value.
Constraint: neq1.
On entry, neq=value and ldysav=value.
Constraint: neqldysav.
On entry, rtol=value.
Constraint: rtol0.0.
On entry, tout is less than t with respect to the direction of integration given by the sign of h0 in a prior call to a setup routine.
tout=value, t=value and h0=value.
On entry, tout is too close to t to start integration.
tout=value and t=value.
The initial stepsize, value, is too small.
Weight number i=value used in the local error test is too small. Check the values of rtol and atol.
atoli and yi may both be zero.
Weight i=value.
ifail=2
At time value the maximum number of allowed steps on this call was taken before reaching the next output point tout=value.
Maximum number of steps =value.
ifail=3
Too much accuracy requested for precision of the machine at time value.
The tolerances should be checked; the requested accuracy should be reduced by a factor of at least value.
With the given values of rtol and atol no further progress can be made across the integration range from the current point t. The components y1,y2,,yneq contain the computed values of the solution at the current point t.
ifail=4
There were repeated error-test failures on an attempted step, before completing the requested task, but the integration was successful as far as t. The problem may have a singularity, or the local error requirements may be inappropriate.
ifail=5
There were repeated convergence-test failures on an attempted step, before completing the requested task, but the integration was successful as far as t. This may be caused by an inaccurate Jacobian matrix or one which is incorrectly computed.
ifail=6
At time value, error weight value became zero. Check the values of atol, rtol and itol supplied.
ifail=7
resid set ires=3, which signals that an error condition has occurred in the solution vector, its time derivative or in the value of t. It was not possible to remove this condition.
ires=value at t=value.
ifail=8
Attempt was made to reduce the step size to a value less than the minimum step size during the calculation of initial values.
Minimum stepsize: value.
Nonlinear solver failed to converge using a damped Newton method to solve for initial values.
Damping factor: value; convergence rate: value.
The residual routine returned an error when calculating the initial values of the solution and its time derivative.
The user problem has one or more inconsistencies between the ires=1 and ires=-1 parts. Integration will not be attempted.
Workspace error occurred when trying to form the Jacobian matrix in calculating the initial values of the solution and its time derivative.
ifail=9
A singular Jacobian has been encountered. You should check the problem formulation and Jacobian calculation.
ifail=10
Larger integer workspace required.
Provided: value; required: value.
Not enough integer store provided for internal storage pointers.
Units of store needed: value. Amount provided: value.
Not enough integer store provided for sparse matrix solver.
Units of store needed: value. Amount provided: value.
Not enough real store provided for sparse matrix solver.
Units of store needed: value. Amount provided: value.
ifail=11
resid set ires=2, which signals that the integration should terminate. ires=value at time value.
ifail=12
A return was forced by setting imon=-2, but the integration was successful as far as t.
ifail=13
The requested task has been completed, but it is estimated that a small change in rtol and atol is unlikely to produce any change in the computed solution. (This ONLY applies when you are NOT operating in one step mode; that is, when itask2 or 5.)
ifail=14
On entry, too much accuracy requested for precision of the machine at the start of problem. The tolerances should be checked; the requested accuracy should be reduced by a factor of at least value.
ifail=15
Either the full matrix linear algebra setup routine was not called first or a communication array has become corrupted.
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.
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.
ifail=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

The accuracy of the numerical solution may be controlled by a careful choice of the arguments rtol and atol, and to a much lesser extent by the choice of norm. You are advised to use scalar error control unless the components of the solution are expected to be poorly scaled. For the type of decaying solution typical of many stiff problems, relative error control with a small absolute error threshold will be most appropriate (that is, you are advised to choose itol=1 with atol1 small but positive).

8 Parallelism and Performance

d02ngf is not thread safe and should not be called from a multithreaded user program. Please see Section 1 in FL Interface Multithreading for more information on thread safety.
d02ngf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
d02ngf 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 implementation-specific information.

9 Further Comments

The cost of computing a solution depends critically on the size of the differential system and to a lesser extent on the degree of stiffness of the problem. For d02ngf the cost is proportional to neq3, though for problems which are only mildly nonlinear the cost may be dominated by factors proportional to neq2 except for very large problems.
In general, you are advised to choose the BDF option (setup routine d02nvf) but if efficiency is of great importance and especially if it is suspected that y A-1g has complex eigenvalues near the imaginary axis for some part of the integration, you should try the BLEND option (setup routine d02nwf).

10 Example

This example solves the well-known stiff Robertson problem written in implicit form
r1 = -0.04a + 1.0E4bc - a r2 = 0.04a - 1.0E4bc - 3.0E7b2 - b r3 = 3.0E7b2 - c  
with initial conditions a=1.0 and b=c=0.0 over the range 0,0.1 with vector error control (itol=4), the BDF method (setup routine d02nvf) and functional iteration. The Jacobian is calculated numerically if the functional iteration encounters difficulty and the integration is in one-step mode (itask=2), with C0 interpolation to calculate the solution at intervals of 0.02 using d02xjf externally. d02nby is used for monitr.

10.1 Program Text

Program Text (d02ngfe.f90)

10.2 Program Data

Program Data (d02ngfe.d)

10.3 Program Results

Program Results (d02ngfe.r)
GnuplotProduced by GNUPLOT 4.6 patchlevel 3 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0 0.02 0.04 0.06 0.08 0.1 0.996 0.9965 0.997 0.9975 0.998 0.9985 0.999 0.9995 1 Solution (100*b,c) Solution (a) x Example Program Stiff DAE with Full Jacobian Stiff Robertson Problem using BDF and Functional Iteration a 100*b c gnuplot_plot_1 gnuplot_plot_2 gnuplot_plot_3