D03PPF/D03PPA (PDF version)
D03 Chapter Contents
D03 Chapter Introduction
NAG Library Manual

NAG Library Routine Document

D03PPF/D03PPA

Note:  before using this routine, please read the Users' Note for your implementation to check the interpretation of bold italicised terms and other implementation-dependent details.

+ Contents

    1  Purpose
    7  Accuracy

1  Purpose

D03PPF/D03PPA integrates a system of linear or nonlinear parabolic partial differential equations (PDEs) in one space variable, with scope for coupled ordinary differential equations (ODEs), and automatic adaptive spatial remeshing. The spatial discretization is performed using finite differences, and the method of lines is employed to reduce the PDEs to a system of ODEs. The resulting system is solved using a Backward Differentiation Formula (BDF) method or a Theta method (switching between Newton's method and functional iteration).
D03PPA is a version of D03PPF that has additional parameters in order to make it safe for use in multithreaded applications (see Section 5).

2  Specification

2.1  Specification for D03PPF

INTEGER  NPDE, M, NPTS, NCODE, NXI, NEQN, ITOL, NXFIX, NRMESH, IPMINF, LRSAVE, ISAVE(LISAVE), LISAVE, ITASK, ITRACE, IND, IFAIL
REAL (KIND=nag_wp)  TS, TOUT, U(NEQN), X(NPTS), XI(NXI), RTOL(*), ATOL(*), ALGOPT(30), XFIX(*), DXMESH, TRMESH, XRATIO, CON, RSAVE(LRSAVE)
LOGICAL  REMESH
CHARACTER(1)  NORM, LAOPT
EXTERNAL  PDEDEF, BNDARY, UVINIT, ODEDEF, MONITF

2.2  Specification for D03PPA

INTEGER  NPDE, M, NPTS, NCODE, NXI, NEQN, ITOL, NXFIX, NRMESH, IPMINF, LRSAVE, ISAVE(LISAVE), LISAVE, ITASK, ITRACE, IND, IUSER(*), IWSAV(505), IFAIL
REAL (KIND=nag_wp)  TS, TOUT, U(NEQN), X(NPTS), XI(NXI), RTOL(*), ATOL(*), ALGOPT(30), XFIX(*), DXMESH, TRMESH, XRATIO, CON, RSAVE(LRSAVE), RUSER(*), RWSAV(1100)
LOGICAL  REMESH, LWSAV(100)
CHARACTER(1)  NORM, LAOPT
CHARACTER(80)  CWSAV(10)
EXTERNAL  PDEDEF, BNDARY, UVINIT, ODEDEF, MONITF

3  Description

D03PPF/D03PPA integrates the system of parabolic-elliptic equations and coupled ODEs
j=1NPDEPi,j Uj t +Qi=x-m x xmRi,  i=1,2,,NPDE ,   axb,tt0, (1)
Fit,V,V.,ξ,U*,Ux*,R*,Ut*,Uxt*=0,  i=1,2,,NCODE, (2)
where (1) defines the PDE part and (2) generalizes the coupled ODE part of the problem.
In (1), Pi,j and Ri depend on x, t, U, Ux, and V; Qi depends on x, t, U, Ux, V and linearly on V.. The vector U is the set of PDE solution values
Ux,t=U1x,t,,UNPDEx,tT,
and the vector Ux is the partial derivative with respect to x. The vector V is the set of ODE solution values
Vt=V1t,,VNCODEtT,
and V. denotes its derivative with respect to time.
In (2), ξ represents a vector of nξ spatial coupling points at which the ODEs are coupled to the PDEs. These points may or may not be equal to some of the PDE spatial mesh points. U*, Ux*, R*, Ut* and Uxt* are the functions U, Ux, R, Ut and Uxt evaluated at these coupling points. Each Fi may only depend linearly on time derivatives. Hence the equation (2) may be written more precisely as
F=G-AV.-B Ut* Uxt* , (3)
where F=F1,,FNCODET, G is a vector of length NCODE, A is an NCODE by NCODE matrix, B is an NCODE by nξ×NPDE matrix and the entries in G, A and B may depend on t, ξ, U*, Ux* and V. In practice you only need to supply a vector of information to define the ODEs and not the matrices A and B. (See Section 5 for the specification of ODEDEF.)
The integration in time is from t0 to tout, over the space interval axb, where a=x1 and b=xNPTS are the leftmost and rightmost points of a mesh x1,x2,,xNPTS defined initially by you and (possibly) adapted automatically during the integration according to user-specified criteria. The coordinate system in space is defined by the following values of m; m=0 for Cartesian coordinates, m=1 for cylindrical polar coordinates and m=2 for spherical polar coordinates.
The PDE system which is defined by the functions Pi,j, Qi and Ri must be specified in PDEDEF.
The initial t=t0 values of the functions Ux,t and Vt must be specified in UVINIT. Note that UVINIT will be called again following any initial remeshing, and so Ux,t0 should be specified for all values of x in the interval axb, and not just the initial mesh points.
The functions Ri which may be thought of as fluxes, are also used in the definition of the boundary conditions. The boundary conditions must have the form
βix,tRix,t,U,Ux,V=γix,t,U,Ux,V,V.,  i=1,2,,NPDE, (4)
where x=a or x=b.
The boundary conditions must be specified in BNDARY. The function γi may depend linearly on V..
The problem is subject to the following restrictions:
(i) In (1), V.jt, for j=1,2,,NCODE, may only appear linearly in the functions Qi, for i=1,2,,NPDE, with a similar restriction for γ;
(ii) Pi,j and the flux Ri must not depend on any time derivatives;
(iii) t0<tout, so that integration is in the forward direction;
(iv) The evaluation of the terms Pi,j, Qi and Ri is done approximately at the mid-points of the mesh Xi, for i=1,2,,NPTS, by calling the PDEDEF for each mid-point in turn. Any discontinuities in these functions must therefore be at one or more of the fixed mesh points specified by XFIX;
(v) At least one of the functions Pi,j must be nonzero so that there is a time derivative present in the PDE problem;
(vi) If m>0 and x1=0.0, which is the left boundary point, then it must be ensured that the PDE solution is bounded at this point. This can be done by either specifying the solution at x=0.0 or by specifying a zero flux there, that is βi=1.0 and γi=0.0. See also Section 8.
The algebraic-differential equation system which is defined by the functions Fi must be specified in ODEDEF. You must also specify the coupling points ξ in the array XI.
The parabolic equations are approximated by a system of ODEs in time for the values of Ui at mesh points. For simple problems in Cartesian coordinates, this system is obtained by replacing the space derivatives by the usual central, three-point finite difference formula. However, for polar and spherical problems, or problems with nonlinear coefficients, the space derivatives are replaced by a modified three-point formula which maintains second order accuracy. In total there are NPDE×NPTS+NCODE ODEs in time direction. This system is then integrated forwards in time using a Backward Differentiation Formula (BDF) or a Theta method.
The adaptive space remeshing can be used to generate meshes that automatically follow the changing time-dependent nature of the solution, generally resulting in a more efficient and accurate solution using fewer mesh points than may be necessary with a fixed uniform or non-uniform mesh. Problems with travelling wavefronts or variable-width boundary layers for example will benefit from using a moving adaptive mesh. The discrete time-step method used here (developed by Furzeland (1984)) automatically creates a new mesh based on the current solution profile at certain time-steps, and the solution is then interpolated onto the new mesh and the integration continues.
The method requires you to supply a MONITF which specifies in an analytical or numerical form the particular aspect of the solution behaviour you wish to track. This so-called monitor function is used to choose a mesh which equally distributes the integral of the monitor function over the domain. A typical choice of monitor function is the second space derivative of the solution value at each point (or some combination of the second space derivatives if there is more than one solution component), which results in refinement in regions where the solution gradient is changing most rapidly.
You must specify the frequency of mesh updates together with certain other criteria such as adjacent mesh ratios. Remeshing can be expensive and you are encouraged to experiment with the different options in order to achieve an efficient solution which adequately tracks the desired features of the solution.
Note that unless the monitor function for the initial solution values is zero at all user-specified initial mesh points, a new initial mesh is calculated and adopted according to the user-specified remeshing criteria. UVINIT will then be called again to determine the initial solution values at the new mesh points (there is no interpolation at this stage) and the integration proceeds.

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 time-dependent problems using the method of lines and differential-algebraic integrators Appl. Numer. Math. 5 375–397
Berzins M and Furzeland R M (1992) An adaptive theta method for the solution of stiff and nonstiff differential equations Appl. Numer. Math. 9 1–19
Furzeland R M (1984) The construction of adaptive space meshes TNER.85.022 Thornton Research Centre, Chester
Skeel R D and Berzins M (1990) A method for the spatial discretization of parabolic equations in one space variable SIAM J. Sci. Statist. Comput. 11(1) 1–32

5  Parameters

1:     NPDE – INTEGERInput
On entry: the number of PDEs to be solved.
Constraint: NPDE1.
2:     M – INTEGERInput
On entry: the coordinate system used:
M=0
Indicates Cartesian coordinates.
M=1
Indicates cylindrical polar coordinates.
M=2
Indicates spherical polar coordinates.
Constraint: M=0, 1 or 2.
3:     TS – REAL (KIND=nag_wp)Input/Output
On entry: the initial value of the independent variable t.
On exit: the value of t corresponding to the solution values in U. Normally TS=TOUT.
Constraint: TS<TOUT.
4:     TOUT – REAL (KIND=nag_wp)Input
On entry: the final value of t to which the integration is to be carried out.
5:     PDEDEF – SUBROUTINE, supplied by the user.External Procedure
PDEDEF must evaluate the functions Pi,j, Qi and Ri which define the system of PDEs. The functions may depend on x, t, U, Ux and V. Qi may depend linearly on V.. PDEDEF is called approximately midway between each pair of mesh points in turn by D03PPF/D03PPA.
The specification of PDEDEF for D03PPF is:
SUBROUTINE PDEDEF ( NPDE, T, X, U, UX, NCODE, V, VDOT, P, Q, R, IRES)
INTEGER  NPDE, NCODE, IRES
REAL (KIND=nag_wp)  T, X, U(NPDE), UX(NPDE), V(NCODE), VDOT(NCODE), P(NPDE,NPDE), Q(NPDE), R(NPDE)
The specification of PDEDEF for D03PPA is:
SUBROUTINE PDEDEF ( NPDE, T, X, U, UX, NCODE, V, VDOT, P, Q, R, IRES, IUSER, RUSER)
INTEGER  NPDE, NCODE, IRES, IUSER(*)
REAL (KIND=nag_wp)  T, X, U(NPDE), UX(NPDE), V(NCODE), VDOT(NCODE), P(NPDE,NPDE), Q(NPDE), R(NPDE), RUSER(*)
1:     NPDE – INTEGERInput
On entry: the number of PDEs in the system.
2:     T – REAL (KIND=nag_wp)Input
On entry: the current value of the independent variable t.
3:     X – REAL (KIND=nag_wp)Input
On entry: the current value of the space variable x.
4:     U(NPDE) – REAL (KIND=nag_wp) arrayInput
On entry: Ui contains the value of the component Uix,t, for i=1,2,,NPDE.
5:     UX(NPDE) – REAL (KIND=nag_wp) arrayInput
On entry: UXi contains the value of the component Uix,t x , for i=1,2,,NPDE.
6:     NCODE – INTEGERInput
On entry: the number of coupled ODEs in the system.
7:     V(NCODE) – REAL (KIND=nag_wp) arrayInput
On entry: if NCODE>0, Vi contains the value of the component Vit, for i=1,2,,NCODE.
8:     VDOT(NCODE) – REAL (KIND=nag_wp) arrayInput
On entry: if NCODE>0, VDOTi contains the value of component V.it, for i=1,2,,NCODE.
Note:  V.it, for i=1,2,,NCODE, may only appear linearly in Qj, for j=1,2,,NPDE.
9:     P(NPDE,NPDE) – REAL (KIND=nag_wp) arrayOutput
On exit: Pij must be set to the value of Pi,jx,t,U,Ux,V, for i=1,2,,NPDE and j=1,2,,NPDE.
10:   Q(NPDE) – REAL (KIND=nag_wp) arrayOutput
On exit: Qi must be set to the value of Qix,t,U,Ux,V,V., for i=1,2,,NPDE.
11:   R(NPDE) – REAL (KIND=nag_wp) arrayOutput
On exit: Ri must be set to the value of Rix,t,U,Ux,V, for i=1,2,,NPDE.
12:   IRES – INTEGERInput/Output
On entry: set to -1​ or ​1.
On exit: should usually remain unchanged. However, you may set IRES to force the integration routine to take certain actions as described below:
IRES=2
Indicates to the integrator that control should be passed back immediately to the calling (sub)routine with the error indicator set to IFAIL=6.
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 IRES=3 when a physically meaningless input or output value has been generated. If you consecutively set IRES=3, then D03PPF/D03PPA returns to the calling subroutine with the error indicator set to IFAIL=4.
Note: the following are additional parameters for specific use with D03PPA. Users of D03PPF therefore need not read the remainder of this description.
13:   IUSER(*) – INTEGER arrayUser Workspace
14:   RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
PDEDEF is called with the parameters IUSER and RUSER as supplied to D03PPF/D03PPA. You are free to use the arrays IUSER and RUSER to supply information to PDEDEF as an alternative to using COMMON global variables.
PDEDEF must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which D03PPF/D03PPA is called. Parameters denoted as Input must not be changed by this procedure.
6:     BNDARY – SUBROUTINE, supplied by the user.External Procedure
BNDARY must evaluate the functions βi and γi which describe the boundary conditions, as given in (4).
The specification of BNDARY for D03PPF is:
SUBROUTINE BNDARY ( NPDE, T, U, UX, NCODE, V, VDOT, IBND, BETA, GAMMA, IRES)
INTEGER  NPDE, NCODE, IBND, IRES
REAL (KIND=nag_wp)  T, U(NPDE), UX(NPDE), V(NCODE), VDOT(NCODE), BETA(NPDE), GAMMA(NPDE)
The specification of BNDARY for D03PPA is:
SUBROUTINE BNDARY ( NPDE, T, U, UX, NCODE, V, VDOT, IBND, BETA, GAMMA, IRES, IUSER, RUSER)
INTEGER  NPDE, NCODE, IBND, IRES, IUSER(*)
REAL (KIND=nag_wp)  T, U(NPDE), UX(NPDE), V(NCODE), VDOT(NCODE), BETA(NPDE), GAMMA(NPDE), RUSER(*)
1:     NPDE – INTEGERInput
On entry: the number of PDEs in the system.
2:     T – REAL (KIND=nag_wp)Input
On entry: the current value of the independent variable t.
3:     U(NPDE) – REAL (KIND=nag_wp) arrayInput
On entry: Ui contains the value of the component Uix,t at the boundary specified by IBND, for i=1,2,,NPDE.
4:     UX(NPDE) – REAL (KIND=nag_wp) arrayInput
On entry: UXi contains the value of the component Uix,t x  at the boundary specified by IBND, for i=1,2,,NPDE.
5:     NCODE – INTEGERInput
On entry: the number of coupled ODEs in the system.
6:     V(NCODE) – REAL (KIND=nag_wp) arrayInput
On entry: if NCODE>0, Vi contains the value of the component Vit, for i=1,2,,NCODE.
7:     VDOT(NCODE) – REAL (KIND=nag_wp) arrayInput
On entry: VDOTi contains the value of component V.it, for i=1,2,,NCODE.
Note: V.it, for i=1,2,,NCODE, may only appear linearly in γj, for j=1,2,,NPDE.
8:     IBND – INTEGERInput
On entry: specifies which boundary conditions are to be evaluated.
IBND=0
BNDARY must set up the coefficients of the left-hand boundary, x=a.
IBND0
BNDARY must set up the coefficients of the right-hand boundary, x=b.
9:     BETA(NPDE) – REAL (KIND=nag_wp) arrayOutput
On exit: BETAi must be set to the value of βix,t at the boundary specified by IBND, for i=1,2,,NPDE.
10:   GAMMA(NPDE) – REAL (KIND=nag_wp) arrayOutput
On exit: GAMMAi must be set to the value of γix,t,U,Ux,V,V. at the boundary specified by IBND, for i=1,2,,NPDE.
11:   IRES – INTEGERInput/Output
On entry: set to -1​ or ​1.
On exit: should usually remain unchanged. However, you may set IRES to force the integration routine to take certain actions as described below:
IRES=2
Indicates to the integrator that control should be passed back immediately to the calling (sub)routine with the error indicator set to IFAIL=6.
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 IRES=3 when a physically meaningless input or output value has been generated. If you consecutively set IRES=3, then D03PPF/D03PPA returns to the calling subroutine with the error indicator set to IFAIL=4.
Note: the following are additional parameters for specific use with D03PPA. Users of D03PPF therefore need not read the remainder of this description.
12:   IUSER(*) – INTEGER arrayUser Workspace
13:   RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
BNDARY is called with the parameters IUSER and RUSER as supplied to D03PPF/D03PPA. You are free to use the arrays IUSER and RUSER to supply information to BNDARY as an alternative to using COMMON global variables.
BNDARY must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which D03PPF/D03PPA is called. Parameters denoted as Input must not be changed by this procedure.
7:     UVINIT – SUBROUTINE, supplied by the user.External Procedure
UVINIT must supply the initial t=t0 values of Ux,t and Vt for all values of x in the interval axb.
The specification of UVINIT for D03PPF is:
SUBROUTINE UVINIT ( NPDE, NPTS, NXI, X, XI, U, NCODE, V)
INTEGER  NPDE, NPTS, NXI, NCODE
REAL (KIND=nag_wp)  X(NPTS), XI(NXI), U(NPDE,NPTS), V(NCODE)
The specification of UVINIT for D03PPA is:
SUBROUTINE UVINIT ( NPDE, NPTS, NXI, X, XI, U, NCODE, V, IUSER, RUSER)
INTEGER  NPDE, NPTS, NXI, NCODE, IUSER(*)
REAL (KIND=nag_wp)  X(NPTS), XI(NXI), U(NPDE,NPTS), V(NCODE), RUSER(*)
1:     NPDE – INTEGERInput
On entry: the number of PDEs in the system.
2:     NPTS – INTEGERInput
On entry: the number of mesh points in the interval a,b.
3:     NXI – INTEGERInput
On entry: the number of ODE/PDE coupling points.
4:     X(NPTS) – REAL (KIND=nag_wp) arrayInput
On entry: the current mesh. Xi contains the value of xi, for i=1,2,,NPTS.
5:     XI(NXI) – REAL (KIND=nag_wp) arrayInput
On entry: if NXI>0, XIi contains the value of the ODE/PDE coupling point, ξi, for i=1,2,,NXI.
6:     U(NPDE,NPTS) – REAL (KIND=nag_wp) arrayOutput
On exit: if NXI>0, Uij contains the value of the component Uixj,t0, for i=1,2,,NPDE and j=1,2,,NPTS.
7:     NCODE – INTEGERInput
On entry: the number of coupled ODEs in the system.
8:     V(NCODE) – REAL (KIND=nag_wp) arrayOutput
On exit: Vi contains the value of component Vit0, for i=1,2,,NCODE.
Note: the following are additional parameters for specific use with D03PPA. Users of D03PPF therefore need not read the remainder of this description.
9:     IUSER(*) – INTEGER arrayUser Workspace
10:   RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
UVINIT is called with the parameters IUSER and RUSER as supplied to D03PPF/D03PPA. You are free to use the arrays IUSER and RUSER to supply information to UVINIT as an alternative to using COMMON global variables.
UVINIT must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which D03PPF/D03PPA is called. Parameters denoted as Input must not be changed by this procedure.
8:     U(NEQN) – REAL (KIND=nag_wp) arrayInput/Output
On entry: if IND=1, the value of U must be unchanged from the previous call.
On exit: UNPDE×j-1+i contains the computed solution Uixj,t, for i=1,2,,NPDE and j=1,2,,NPTS, and UNPTS×NPDE+k contains Vkt, for k=1,2,,NCODE, evaluated at t=TS.
9:     NPTS – INTEGERInput
On entry: the number of mesh points in the interval a,b.
Constraint: NPTS3.
10:   X(NPTS) – REAL (KIND=nag_wp) arrayInput/Output
On entry: the initial mesh points in the space direction. X1 must specify the left-hand boundary, a, and XNPTS must specify the right-hand boundary, b.
Constraint: X1<X2<<XNPTS.
On exit: the final values of the mesh points.
11:   NCODE – INTEGERInput
On entry: the number of coupled ODE in the system.
Constraint: NCODE0.
12:   ODEDEF – SUBROUTINE, supplied by the NAG Library or the user.External Procedure
ODEDEF must evaluate the functions F, which define the system of ODEs, as given in (3).
If you wish to compute the solution of a system of PDEs only (NCODE=0), ODEDEF must be the dummy routine D03PCK/D53PCK for D03PPF (or D53PCK for D03PPA). D03PCK/D53PCK and D53PCK are included in the NAG Library.
The specification of ODEDEF for D03PPF is:
SUBROUTINE ODEDEF ( NPDE, T, NCODE, V, VDOT, NXI, XI, UCP, UCPX, RCP, UCPT, UCPTX, F, IRES)
INTEGER  NPDE, NCODE, NXI, IRES
REAL (KIND=nag_wp)  T, V(NCODE), VDOT(NCODE), XI(NXI), UCP(NPDE,*), UCPX(NPDE,*), RCP(NPDE,*), UCPT(NPDE,*), UCPTX(NPDE,*), F(NCODE)
The specification of ODEDEF for D03PPA is:
SUBROUTINE ODEDEF ( NPDE, T, NCODE, V, VDOT, NXI, XI, UCP, UCPX, RCP, UCPT, UCPTX, F, IRES, IUSER, RUSER)
INTEGER  NPDE, NCODE, NXI, IRES, IUSER(*)
REAL (KIND=nag_wp)  T, V(NCODE), VDOT(NCODE), XI(NXI), UCP(NPDE,*), UCPX(NPDE,*), RCP(NPDE,*), UCPT(NPDE,*), UCPTX(NPDE,*), F(NCODE), RUSER(*)
1:     NPDE – INTEGERInput
On entry: the number of PDEs in the system.
2:     T – REAL (KIND=nag_wp)Input
On entry: the current value of the independent variable t.
3:     NCODE – INTEGERInput
On entry: the number of coupled ODEs in the system.
4:     V(NCODE) – REAL (KIND=nag_wp) arrayInput
On entry: if NCODE>0, Vi contains the value of the component Vit, for i=1,2,,NCODE.
5:     VDOT(NCODE) – REAL (KIND=nag_wp) arrayInput
On entry: if NCODE>0, VDOTi contains the value of component V.it, for i=1,2,,NCODE.
6:     NXI – INTEGERInput
On entry: the number of ODE/PDE coupling points.
7:     XI(NXI) – REAL (KIND=nag_wp) arrayInput
On entry: if NXI>0, XIi contains the ODE/PDE coupling points, ξi, for i=1,2,,NXI.
8:     UCP(NPDE,*) – REAL (KIND=nag_wp) arrayInput
On entry: if NXI>0, UCPij contains the value of Uix,t at the coupling point x=ξj, for i=1,2,,NPDE and j=1,2,,NXI.
9:     UCPX(NPDE,*) – REAL (KIND=nag_wp) arrayInput
On entry: if NXI>0, UCPXij contains the value of Uix,t x  at the coupling point x=ξj, for i=1,2,,NPDE and j=1,2,,NXI.
10:   RCP(NPDE,*) – REAL (KIND=nag_wp) arrayInput
On entry: RCPij contains the value of the flux Ri at the coupling point x=ξj, for i=1,2,,NPDE and j=1,2,,NXI.
11:   UCPT(NPDE,*) – REAL (KIND=nag_wp) arrayInput
On entry: if NXI>0, UCPTij contains the value of Ui t  at the coupling point x=ξj, for i=1,2,,NPDE and j=1,2,,NXI.
12:   UCPTX(NPDE,*) – REAL (KIND=nag_wp) arrayInput
On entry: UCPTXij contains the value of 2Ui x t  at the coupling point x=ξj, for i=1,2,,NPDE and j=1,2,,NXI.
13:   F(NCODE) – REAL (KIND=nag_wp) arrayOutput
On exit: Fi must contain the ith component of F, for i=1,2,,NCODE, where F is defined as
F=G-AV.-B Ut* Uxt* , (5)
or
F=-AV.-B Ut* Uxt* . (6)
The definition of F is determined by the input value of IRES.
14:   IRES – INTEGERInput/Output
On entry: the form of F that must be returned in the array F.
IRES=1
Equation (5) must be used.
IRES=-1
Equation (6) must be used.
On exit: should usually remain unchanged. However, you may reset IRES to force the integration routine to take certain actions as described below:
IRES=2
Indicates to the integrator that control should be passed back immediately to the calling (sub)routine with the error indicator set to IFAIL=6.
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 IRES=3 when a physically meaningless input or output value has been generated. If you consecutively set IRES=3, then D03PPF/D03PPA returns to the calling subroutine with the error indicator set to IFAIL=4.
Note: the following are additional parameters for specific use with D03PPA. Users of D03PPF therefore need not read the remainder of this description.
15:   IUSER(*) – INTEGER arrayUser Workspace
16:   RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
ODEDEF is called with the parameters IUSER and RUSER as supplied to D03PPF/D03PPA. You are free to use the arrays IUSER and RUSER to supply information to ODEDEF as an alternative to using COMMON global variables.
ODEDEF must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which D03PPF/D03PPA is called. Parameters denoted as Input must not be changed by this procedure.
13:   NXI – INTEGERInput
On entry: the number of ODE/PDE coupling points.
Constraints:
  • if NCODE=0, NXI=0;
  • if NCODE>0, NXI0.
14:   XI(NXI) – REAL (KIND=nag_wp) arrayInput
On entry: if NXI>0, XIi, for i=1,2,,NXI, must be set to the ODE/PDE coupling points.
Constraint: X1XI1<XI2<<XINXIXNPTS.
15:   NEQN – INTEGERInput
On entry: the number of ODEs in the time direction.
Constraint: NEQN=NPDE×NPTS+NCODE.
16:   RTOL(*) – REAL (KIND=nag_wp) arrayInput
Note: the dimension of the array RTOL must be at least 1 if ITOL=1 or 2 and at least NEQN if ITOL=3 or 4.
On entry: the relative local error tolerance.
Constraint: RTOLi0.0 for all relevant i.
17:   ATOL(*) – REAL (KIND=nag_wp) arrayInput
Note: the dimension of the array ATOL must be at least 1 if ITOL=1 or 3 and at least NEQN if ITOL=2 or 4.
On entry: the absolute local error tolerance.
Constraints:
  • ATOLi0.0 for all relevant i;
  • Corresponding elements of ATOL and RTOL cannot both be 0.0.
18:   ITOL – INTEGERInput
On entry: a value to indicate the form of the local error test. ITOL indicates to D03PPF/D03PPA whether to interpret either or both of RTOL or ATOL as a vector or scalar. The error test to be satisfied is ei/wi<1.0, where wi is defined as follows:
ITOLRTOLATOLwi
1scalarscalarRTOL1×Ui+ATOL1
2scalarvectorRTOL1×Ui+ATOLi
3vectorscalarRTOLi×Ui+ATOL1
4vectorvectorRTOLi×Ui+ATOLi
In the above, ei denotes the estimated local error for the ith component of the coupled PDE/ODE system in time, Ui, for i=1,2,,NEQN.
The choice of norm used is defined by the parameter NORM.
Constraint: 1ITOL4.
19:   NORM – CHARACTER(1)Input
On entry: the type of norm to be used.
NORM='M'
Maximum norm.
NORM='A'
Averaged L2 norm.
If Unorm denotes the norm of the vector U of length NEQN, then for the averaged L2 norm
Unorm=1NEQNi=1NEQNUi/wi2,
while for the maximum norm
U norm = maxi Ui / wi .
See the description of ITOL for the formulation of the weight vector w.
Constraint: NORM='M' or 'A'.
20:   LAOPT – CHARACTER(1)Input
On entry: the type of matrix algebra required.
LAOPT='F'
Full matrix methods to be used.
LAOPT='B'
Banded matrix methods to be used.
LAOPT='S'
Sparse matrix methods to be used.
Constraint: LAOPT='F', 'B' or 'S'.
Note: you are recommended to use the banded option when no coupled ODEs are present (i.e., NCODE=0).
21:   ALGOPT(30) – REAL (KIND=nag_wp) arrayInput
On entry: may be set to control various options available in the integrator. If you wish to employ all the default options, then ALGOPT1 should be set to 0.0. Default values will also be used for any other elements of ALGOPT set to zero. The permissible values, default values, and meanings are as follows:
ALGOPT1
Selects the ODE integration method to be used. If ALGOPT1=1.0, a BDF method is used and if ALGOPT1=2.0, a Theta method is used. The default value is ALGOPT1=1.0.
If ALGOPT1=2.0, then ALGOPTi, for i=2,3,4 are not used.
ALGOPT2
Specifies the maximum order of the BDF integration formula to be used. ALGOPT2 may be 1.0, 2.0, 3.0, 4.0 or 5.0. The default value is ALGOPT2=5.0.
ALGOPT3
Specifies what method is to be used to solve the system of nonlinear equations arising on each step of the BDF method. If ALGOPT3=1.0 a modified Newton iteration is used and if ALGOPT3=2.0 a functional iteration method is used. If functional iteration is selected and the integrator encounters difficulty, then there is an automatic switch to the modified Newton iteration. The default value is ALGOPT3=1.0.
ALGOPT4
Specifies whether or not the Petzold error test is to be employed. The Petzold error test results in extra overhead but is more suitable when algebraic equations are present, such as Pi,j=0.0, for j=1,2,,NPDE, for some i or when there is no V.it dependence in the coupled ODE system. If ALGOPT4=1.0, then the Petzold test is used. If ALGOPT4=2.0, then the Petzold test is not used. The default value is ALGOPT4=1.0.
If ALGOPT1=1.0, then ALGOPTi, for i=5,6,7, are not used.
ALGOPT5
Specifies the value of Theta to be used in the Theta integration method. 0.51ALGOPT50.99. The default value is ALGOPT5=0.55.
ALGOPT6
Specifies what method is to be used to solve the system of nonlinear equations arising on each step of the Theta method. If ALGOPT6=1.0, a modified Newton iteration is used and if ALGOPT6=2.0, a functional iteration method is used. The default value is ALGOPT6=1.0.
ALGOPT7
Specifies whether or not the integrator is allowed to switch automatically between modified Newton and functional iteration methods in order to be more efficient. If ALGOPT7=1.0, then switching is allowed and if ALGOPT7=2.0, then switching is not allowed. The default value is ALGOPT7=1.0.
ALGOPT11
Specifies a point in the time direction, tcrit, beyond which integration must not be attempted. The use of tcrit is described under the parameter ITASK. If ALGOPT10.0, a value of 0.0 for ALGOPT11, say, should be specified even if ITASK subsequently specifies that tcrit will not be used.
ALGOPT12
Specifies the minimum absolute step size to be allowed in the time integration. If this option is not required, ALGOPT12 should be set to 0.0.
ALGOPT13
Specifies the maximum absolute step size to be allowed in the time integration. If this option is not required, ALGOPT13 should be set to 0.0.
ALGOPT14
Specifies the initial step size to be attempted by the integrator. If ALGOPT14=0.0, then the initial step size is calculated internally.
ALGOPT15
Specifies the maximum number of steps to be attempted by the integrator in any one call. If ALGOPT15=0.0, then no limit is imposed.
ALGOPT23
Specifies what method is to be used to solve the nonlinear equations at the initial point to initialize the values of U, Ut, V and V.. If ALGOPT23=1.0, a modified Newton iteration is used and if ALGOPT23=2.0, functional iteration is used. The default value is ALGOPT23=1.0.
ALGOPT29 and ALGOPT30 are used only for the sparse matrix algebra option, LAOPT='S'.
ALGOPT29
Governs the choice of pivots during the decomposition of the first Jacobian matrix. It should lie in the range 0.0<ALGOPT29<1.0, with smaller values biasing the algorithm towards maintaining sparsity at the expense of numerical stability. If ALGOPT29 lies outside this range then the default value is used. If the routines regard the Jacobian matrix as numerically singular then increasing ALGOPT29 towards 1.0 may help, but at the cost of increased fill-in. The default value is ALGOPT29=0.1.
ALGOPT30
Is used as a relative pivot threshold during subsequent Jacobian decompositions (see ALGOPT29) below which an internal error is invoked. If ALGOPT30 is greater than 1.0 no check is made on the pivot size, and this may be a necessary option if the Jacobian is found to be numerically singular (see ALGOPT29). The default value is ALGOPT30=0.0001.
22:   REMESH – LOGICALInput
On entry: indicates whether or not spatial remeshing should be performed.
REMESH=.TRUE.
Indicates that spatial remeshing should be performed as specified.
REMESH=.FALSE.
Indicates that spatial remeshing should be suppressed.
Note:  REMESH should not be changed between consecutive calls to D03PPF/D03PPA. Remeshing can be switched off or on at specified times by using appropriate values for the parameters NRMESH and TRMESH at each call.
23:   NXFIX – INTEGERInput
On entry: the number of fixed mesh points.
Constraint: 0NXFIXNPTS-2.
Note: the end points X1 and XNPTS are fixed automatically and hence should not be specified as fixed points.
24:   XFIX(*) – REAL (KIND=nag_wp) arrayInput
Note: the dimension of the array XFIX must be at least max1,NXFIX.
On entry: XFIXi, for i=1,2,,NXFIX, must contain the value of the x coordinate at the ith fixed mesh point.
Constraints:
  • XFIXi<XFIXi+1, for i=1,2,,NXFIX-1;
  • each fixed mesh point must coincide with a user-supplied initial mesh point, that is XFIXi=Xj for some j, 2jNPTS-1.
Note: the positions of the fixed mesh points in the array X remain fixed during remeshing, and so the number of mesh points between adjacent fixed points (or between fixed points and end points) does not change. You should take this into account when choosing the initial mesh distribution.
25:   NRMESH – INTEGERInput
On entry: specifies the spatial remeshing frequency and criteria for the calculation and adoption of a new mesh.
NRMESH<0
Indicates that a new mesh is adopted according to the parameter DXMESH. The mesh is tested every NRMESH timesteps.
NRMESH=0
Indicates that remeshing should take place just once at the end of the first time step reached when t>TRMESH.
NRMESH>0
Indicates that remeshing will take place every NRMESH time steps, with no testing using DXMESH.
Note: NRMESH may be changed between consecutive calls to D03PPF/D03PPA to give greater flexibility over the times of remeshing.
26:   DXMESH – REAL (KIND=nag_wp)Input
On entry: determines whether a new mesh is adopted when NRMESH is set less than zero. A possible new mesh is calculated at the end of every NRMESH time steps, but is adopted only if
xinew>xi old +DXMESH×xi+1 old -xi old
or
xinew<xi old -DXMESH×xi old -xi- 1 old
DXMESH thus imposes a lower limit on the difference between one mesh and the next.
Constraint: DXMESH0.0.
27:   TRMESH – REAL (KIND=nag_wp)Input
On entry: specifies when remeshing will take place when NRMESH is set to zero. Remeshing will occur just once at the end of the first time step reached when t is greater than TRMESH.
Note: TRMESH may be changed between consecutive calls to D03PPF/D03PPA to force remeshing at several specified times.
28:   IPMINF – INTEGERInput
On entry: the level of trace information regarding the adaptive remeshing. Details are directed to the current advisory message unit (see X04ABF).
IPMINF=0
No trace information.
IPMINF=1
Brief summary of mesh characteristics.
IPMINF=2
More detailed information, including old and new mesh points, mesh sizes and monitor function values.
Constraint: IPMINF=0, 1 or 2.
29:   XRATIO – REAL (KIND=nag_wp)Input
On entry: an input bound on the adjacent mesh ratio (greater than 1.0 and typically in the range 1.5 to 3.0). The remeshing routines will attempt to ensure that
xi-xi-1/XRATIO<xi+1-xi<XRATIO×xi-xi-1.
Suggested value: XRATIO=1.5.
Constraint: XRATIO>1.0.
30:   CON – REAL (KIND=nag_wp)Input
On entry: an input bound on the sub-integral of the monitor function Fmonx over each space step. The remeshing routines will attempt to ensure that
xixi+1FmonxdxCONx1xNPTSFmonxdx,
(see Furzeland (1984)). CON gives you more control over the mesh distribution e.g., decreasing CON allows more clustering. A typical value is 2/NPTS-1, but you are encouraged to experiment with different values. Its value is not critical and the mesh should be qualitatively correct for all values in the range given below.
Suggested value: CON=2.0/NPTS-1.
Constraint: 0.1/NPTS-1CON10.0/NPTS-1.
31:   MONITF – SUBROUTINE, supplied by the NAG Library or the user.External Procedure
MONITF must supply and evaluate a remesh monitor function to indicate the solution behaviour of interest.
If you specify REMESH=.FALSE., i.e., no remeshing, then MONITF will not be called and the dummy routine D03PCL/D53PCL for D03PPF (or D53PCL for D03PPA) may be used for MONITF. (D03PCL/D53PCL and D53PCL are included in the NAG Library.)
The specification of MONITF for D03PPF is:
SUBROUTINE MONITF ( T, NPTS, NPDE, X, U, R, FMON)
INTEGER  NPTS, NPDE
REAL (KIND=nag_wp)  T, X(NPTS), U(NPDE,NPTS), R(NPDE,NPTS), FMON(NPTS)
The specification of MONITF for D03PPA is:
SUBROUTINE MONITF ( T, NPTS, NPDE, X, U, R, FMON, IUSER, RUSER)
INTEGER  NPTS, NPDE, IUSER(*)
REAL (KIND=nag_wp)  T, X(NPTS), U(NPDE,NPTS), R(NPDE,NPTS), FMON(NPTS), RUSER(*)
1:     T – REAL (KIND=nag_wp)Input
On entry: the current value of the independent variable t.
2:     NPTS – INTEGERInput
On entry: the number of mesh points in the interval a,b.
3:     NPDE – INTEGERInput
On entry: the number of PDEs in the system.
4:     X(NPTS) – REAL (KIND=nag_wp) arrayInput
On entry: the current mesh. Xi contains the value of xi, for i=1,2,,NPTS.
5:     U(NPDE,NPTS) – REAL (KIND=nag_wp) arrayInput
On entry: Uij contains the value of Uix,t at x=Xj and time t, for i=1,2,,NPDE and j=1,2,,NPTS.
6:     R(NPDE,NPTS) – REAL (KIND=nag_wp) arrayInput
On entry: Rij contains the value of Rix,t,U,Ux,V at x=Xj and time t, for i=1,2,,NPDE and j=1,2,,NPTS.
7:     FMON(NPTS) – REAL (KIND=nag_wp) arrayOutput
On exit: FMONi must contain the value of the monitor function Fmonx at mesh point x=Xi.
Constraint: FMONi0.0.
Note: the following are additional parameters for specific use with D03PPA. Users of D03PPF therefore need not read the remainder of this description.
8:     IUSER(*) – INTEGER arrayUser Workspace
9:     RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
MONITF is called with the parameters IUSER and RUSER as supplied to D03PPF/D03PPA. You are free to use the arrays IUSER and RUSER to supply information to MONITF as an alternative to using COMMON global variables.
MONITF must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which D03PPF/D03PPA is called. Parameters denoted as Input must not be changed by this procedure.
32:   RSAVE(LRSAVE) – REAL (KIND=nag_wp) arrayCommunication Array
If IND=0, RSAVE need not be set on entry.
If IND=1, RSAVE must be unchanged from the previous call to the routine because it contains required information about the iteration.
33:   LRSAVE – INTEGERInput
On entry: the dimension of the array RSAVE as declared in the (sub)program from which D03PPF/D03PPA is called. Its size depends on the type of matrix algebra selected.
If LAOPT='F', LRSAVENEQN×NEQN+NEQN+nwkres+lenode.
If LAOPT='B', LRSAVE3×mlu+1×NEQN+nwkres+lenode.
If LAOPT='S', LRSAVE4×NEQN+11×NEQN/2+1+nwkres+lenode.
Where
mlu is the lower or upper half bandwidths such that
mlu=2×NPDE-1, for PDE problems only; or
mlu=NEQN-1, for coupled PDE/ODE problems.
nwkres= NPDE×3×NPDE+6×NXI+NPTS+15+NXI+NCODE+7×NPTS+NXFIX+1, when ​NCODE>0​ and ​NXI>0; NPDE×3×NPDE+NPTS+21+NCODE+7×NPTS+NXFIX+2, when ​NCODE>0​ and ​NXI=0; or NPDE×3×NPDE+NPTS+21+7×NPTS+NXFIX+3, when ​NCODE=0.  
lenode= 6+intALGOPT2×NEQN+50, when the BDF method is used; 9×NEQN+50, when the Theta method is used.  
Note: when using the sparse option, the value of LRSAVE may be too small when supplied to the integrator. An estimate of the minimum size of LRSAVE is printed on the current error message unit if ITRACE>0 and the routine returns with IFAIL=15.
34:   ISAVE(LISAVE) – INTEGER arrayCommunication Array
If IND=0, ISAVE need not be set on entry.
If IND=1, ISAVE must be unchanged from the previous call to the routine because it contains required information about the iteration required for subsequent calls. In particular:
ISAVE1
Contains the number of steps taken in time.
ISAVE2
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.
ISAVE3
Contains the number of Jacobian evaluations performed by the time integrator.
ISAVE4
Contains the order of the ODE method last used in the time integration.
ISAVE5
Contains the number of Newton iterations performed by the time integrator. Each iteration involves residual evaluation of the resulting ODE system followed by a back-substitution using the LU decomposition of the Jacobian matrix.
The rest of the array is used as workspace.
35:   LISAVE – INTEGERInput
On entry: the dimension of the array ISAVE as declared in the (sub)program from which D03PPF/D03PPA is called.
Its size depends on the type of matrix algebra selected:
  • if LAOPT='B', LISAVENEQN+25+NXFIX;
  • if LAOPT='F', LISAVE25+NXFIX;
  • if LAOPT='S', LISAVE25×NEQN+25+NXFIX.
Note:  when using the sparse option, the value of LISAVE may be too small when supplied to the integrator. An estimate of the minimum size of LISAVE is printed on the current error message unit if ITRACE>0 and the routine returns with IFAIL=15.
36:   ITASK – INTEGERInput
On entry: specifies the task to be performed by the ODE integrator.
ITASK=1
Normal computation of output values U at t=TOUT.
ITASK=2
One step and return.
ITASK=3
Stop at first internal integration point at or beyond t=TOUT.
ITASK=4
Normal computation of output values U at t=TOUT but without overshooting t=tcrit where tcrit is described under the parameter ALGOPT.
ITASK=5
Take one step in the time direction and return, without passing tcrit, where tcrit is described under the parameter ALGOPT.
Constraint: ITASK=1, 2, 3, 4 or 5.
37:   ITRACE – INTEGERInput
On entry: the level of trace information required from D03PPF/D03PPA and the underlying ODE solver:
ITRACE-1
No output is generated.
ITRACE=0
Only warning messages from the PDE solver are printed on the current error message unit (see X04AAF).
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.
ITRACE=2
Output from the underlying ODE solver is similar to that produced when ITRACE=1, except that the advisory messages are given in greater detail.
ITRACE3
Output from the underlying ODE solver is similar to that produced when ITRACE=2, except that the advisory messages are given in greater detail.
You are advised to set ITRACE=0, unless you are experienced with sub-chapter D02M–N.
38:   IND – INTEGERInput/Output
On entry: must be set to 0 or 1.
IND=0
Starts or restarts the integration in time.
IND=1
Continues the integration after an earlier exit from the routine. In this case, only the parameters TOUT and IFAIL and the remeshing parameters NRMESH, DXMESH, TRMESH, XRATIO and CON may be reset between calls to D03PPF/D03PPA.
Constraint: 0IND1.
On exit: IND=1.
39:   IFAIL – INTEGERInput/Output
Note: for D03PPA, IFAIL does not occur in this position in the parameter list. See the additional parameters described below.
On entry: IFAIL must be set to 0, -1​ or ​1. If you are unfamiliar with this parameter you should refer to Section 3.3 in the Essential Introduction 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, if you are not familiar with this parameter, the recommended value is 0. 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).
Note: the following are additional parameters for specific use with D03PPA. Users of D03PPF therefore need not read the remainder of this description.
39:   IUSER(*) – INTEGER arrayUser Workspace
40:   RUSER(*) – REAL (KIND=nag_wp) arrayUser Workspace
IUSER and RUSER are not used by D03PPF/D03PPA, but are passed directly to PDEDEF, BNDARY, UVINIT, ODEDEF and MONITF and may be used to pass information to these routines as an alternative to using COMMON global variables.
41:   CWSAV(10) – CHARACTER(80) arrayCommunication Array
42:   LWSAV(100) – LOGICAL arrayCommunication Array
43:   IWSAV(505) – INTEGER arrayCommunication Array
44:   RWSAV(1100) – REAL (KIND=nag_wp) arrayCommunication Array
45:   IFAIL – INTEGERInput/Output
Note: see the parameter description for IFAIL above.

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
On entry,TOUT-TS is too small,
orITASK1, 2, 3, 4 or 5,
orM0, 1 or 2,
orat least one of the coupling points defined in array XI is outside the interval X1,XNPTS,
orM>0 and X1<0.0,
orNPTS<3,
orNPDE<1,
orNORM'A' or 'M',
orLAOPT'F', 'B' or 'S',
orITOL1, 2, 3 or 4,
orIND0 or 1,
ormesh points Xi are badly ordered,
orLRSAVE is too small,
orLISAVE is too small,
orNCODE and NXI are incorrectly defined,
oran element of RTOL or ATOL<0.0,
orcorresponding elements of RTOL and ATOL are both 0.0,
orNEQNNPDE×NPTS+NCODE,
orNXFIX not in the range 0 to NPTS-2,
orfixed mesh point(s) do not coincide with any of the user-supplied mesh points,
orDXMESH<0.0,
orIPMINF0, 1 or 2,
orXRATIO1.0,
orCON not in the range 0.1/NPTS-1 to 10/NPTS-1.
IFAIL=2
The underlying ODE solver cannot make any further progress, with the values of ATOL and RTOL, across the integration range from the current point t=TS. The components of U contain the computed values at the current point t=TS.
IFAIL=3
In the underlying ODE solver, there were repeated error test failures on an attempted step, before completing the requested task, but the integration was successful as far as t=TS. The problem may have a singularity, or the error requirement may be inappropriate.
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 at least PDEDEF, BNDARY or ODEDEF, when the residual in the underlying ODE solver was being evaluated.
IFAIL=5
In solving the ODE system, a singular Jacobian has been encountered. You should check your problem formulation.
IFAIL=6
When evaluating the residual in solving the ODE system, IRES was set to 2 in at least PDEDEF, BNDARY or ODEDEF. Integration was successful as far as t=TS.
IFAIL=7
The values of ATOL and RTOL are so small that the routine is unable to start the integration in time.
IFAIL=8
In one of PDEDEF, BNDARY or ODEDEF, IRES was set to an invalid value.
IFAIL=9 (D02NNF)
A serious error has occurred in an internal call to the specified routine. Check the problem specification and all parameters and array dimensions. Setting ITRACE=1 may provide more information. If the problem persists, contact NAG.
IFAIL=10
The required task has been completed, but it is estimated that a small change in ATOL and RTOL is unlikely to produce any change in the computed solution. (Only applies when you are not operating in one step mode, that is when ITASK2 or 5.)
IFAIL=11
An error occurred during Jacobian formulation of the ODE system (a more detailed error description may be directed to the current error message unit). If using the sparse matrix algebra option, the values of ALGOPT29 and ALGOPT30 may be inappropriate.
IFAIL=12
In solving the ODE system, the maximum number of steps specified in ALGOPT15 have been taken.
IFAIL=13
Some error weights wi became zero during the time integration (see the description of ITOL). Pure relative error control (ATOLi=0.0) was requested on a variable (the ith) which has become zero. The integration was successful as far as t=TS.
IFAIL=14
The flux function Ri was detected as depending on time derivatives, which is not permissible.
IFAIL=15
When using the sparse option, the value of LISAVE or LRSAVE was not sufficient (more detailed information may be directed to the current error message unit).
IFAIL=16
REMESH has been changed between calls to D03PPF/D03PPA, which is not permissible.
IFAIL=17
The remeshing process has produced zero or negative mesh spacing. You are advised to check MONITF and to try adjusting the values of DXMESH, XRATIO and CON. Setting IPMINF=1 may provide more information.

7  Accuracy

D03PPF/D03PPA 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 parameters, ATOL and RTOL.

8  Further Comments

The parameter specification allows you to include equations with only first-order derivatives in the space direction but there is no guarantee that the method of integration will be satisfactory for such systems. The position and nature of the boundary conditions in particular are critical in defining a stable problem. It may be advisable in such cases to reduce the whole system to first-order and to use the Keller box scheme routine D03PRF.
The time taken depends on the complexity of the parabolic system, the accuracy requested, and the frequency of the mesh updates. For a given system with fixed accuracy and mesh-update frequency it is approximately proportional to NEQN.

9  Example

This example uses Burgers Equation, a common test problem for remeshing algorithms, given by
U t =-U U x +E 2U x2 ,
for x0,1 and t0,1, where E is a small constant.
The initial and boundary conditions are given by the exact solution
Ux,t=0.1exp-A+0.5exp-B+exp-C exp-A+exp-B+exp-C ,
where
A = 50Ex- 0.5+ 4.95t, B = 250Ex- 0.5+ 0.75t, C = 500Ex- 0.375.

9.1  Program Text

Note: the following programs illustrate the use of D03PPF and D03PPA.

Program Text (d03ppfe.f90)

Program Text (d03ppae.f90)

9.2  Program Data

Program Data (d03ppfe.d)

Program Data (d03ppae.d)

9.3  Program Results

Program Results (d03ppfe.r)

Program Results (d03ppae.r)

Produced by GNUPLOT 4.4 patchlevel 0 Example Program Solution of Burgers Equation using Moving Mesh U(x,t) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time 0 0.2 0.4 0.6 0.8 1 x 0.1 0.5 1

D03PPF/D03PPA (PDF version)
D03 Chapter Contents
D03 Chapter Introduction
NAG Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2012