NAG Library Routine Document

D03PHF/D03PHA

1  Purpose

2  Specification

2.1  Specification for D03PHF

2.2  Specification for D03PHA

3  Description

4  References

5  Parameters

1:     NPDE – INTEGERInput

On entry: the number of PDEs to be solved.

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

2:     M – INTEGERInput

On entry: the coordinate system used:

$\mathbf{M}=0$

Indicates Cartesian coordinates.

$\mathbf{M}=1$

Indicates cylindrical polar coordinates.

$\mathbf{M}=2$

Indicates spherical polar coordinates.

Constraint: $\mathbf{M}=0$, $1$ 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 $\mathbf{TS}=\mathbf{TOUT}$.

Constraint: $\mathbf{TS}<\mathbf{TOUT}$.

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 $P_{i,j}$, $Q_i$ and $R_i$ which define the system of PDEs. The functions may depend on $x$, $t$, $U$, $U_x$ and $V$. $Q_i$ may depend linearly on $\stackrel{.}{V}$. PDEDEF is called approximately midway between each pair of mesh points in turn by D03PHF/D03PHA.

The specification of PDEDEF for D03PHF 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 D03PHA 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: $\mathbf{U}\left(\mathit{i}\right)$ contains the value of the component $U_{\mathit{i}}\left(x,t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

5:     UX(NPDE) – 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 $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

6:     NCODE – INTEGERInput

On entry: the number of coupled ODEs in the system.

7:     V(NCODE) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NCODE}>0$, $\mathbf{V}\left(\mathit{i}\right)$ contains the value of the component $V_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

8:     VDOT(NCODE) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NCODE}>0$, $\mathbf{VDOT}\left(\mathit{i}\right)$ contains the value of component ${\stackrel{.}{V}}_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

Note:  ${\stackrel{.}{V}}_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$, may only appear linearly in $Q_{\mathit{j}}$, for $\mathit{j}=1,2,\dots ,\mathbf{NPDE}$.

9:     P(NPDE,NPDE) – REAL (KIND=nag_wp) arrayOutput

On exit: $\mathbf{P}\left(\mathit{i},\mathit{j}\right)$ must be set to the value of $P_{\mathit{i},\mathit{j}}\left(x,t,U,U_x,V\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NPDE}$.

10:   Q(NPDE) – REAL (KIND=nag_wp) arrayOutput

On exit: $\mathbf{Q}\left(\mathit{i}\right)$ must be set to the value of $Q_{\mathit{i}}\left(x,t,U,U_x,V,\stackrel{.}{V}\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

11:   R(NPDE) – REAL (KIND=nag_wp) arrayOutput

On exit: $\mathbf{R}\left(\mathit{i}\right)$ must be set to the value of $R_{\mathit{i}}\left(x,t,U,U_x,V\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

12:   IRES – INTEGERInput/Output

On entry: set to $-1\text{ or }1$.

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

$\mathbf{IRES}=2$

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 $\mathbf{IRES}=3$, then D03PHF/D03PHA returns to the calling subroutine with the error indicator set to $\mathbf{IFAIL}=\mathbf{4}$.

Note: the following are additional parameters for specific use with D03PHA. Users of D03PHF 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 D03PHF/D03PHA. 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 D03PHF/D03PHA 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 ${\beta }_i$ and ${\gamma }_i$ which describe the boundary conditions, as given in (4).

The specification of BNDARY for D03PHF 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 D03PHA 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: $\mathbf{U}\left(\mathit{i}\right)$ contains the value of the component $U_{\mathit{i}}\left(x,t\right)$ at the boundary specified by IBND, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

4:     UX(NPDE) – REAL (KIND=nag_wp) arrayInput

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

5:     NCODE – INTEGERInput

On entry: the number of coupled ODEs in the system.

6:     V(NCODE) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NCODE}>0$, $\mathbf{V}\left(\mathit{i}\right)$ contains the value of the component $V_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

7:     VDOT(NCODE) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NCODE}>0$, $\mathbf{VDOT}\left(\mathit{i}\right)$ contains the value of component ${\stackrel{.}{V}}_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

Note:  ${\stackrel{.}{V}}_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$, may only appear linearly in $Q_{\mathit{j}}$, for $\mathit{j}=1,2,\dots ,\mathbf{NPDE}$.

8:     IBND – INTEGERInput

On entry: specifies which boundary conditions are to be evaluated.

$\mathbf{IBND}=0$

BNDARY must set up the coefficients of the left-hand boundary, $x=a$.

$\mathbf{IBND}\ne 0$

BNDARY must set up the coefficients of the right-hand boundary, $x=b$.

9:     BETA(NPDE) – REAL (KIND=nag_wp) arrayOutput

On exit: $\mathbf{BETA}\left(\mathit{i}\right)$ must be set to the value of ${\beta }_{\mathit{i}}\left(x,t\right)$ at the boundary specified by IBND, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

10:   GAMMA(NPDE) – REAL (KIND=nag_wp) arrayOutput

On exit: $\mathbf{GAMMA}\left(\mathit{i}\right)$ must be set to the value of ${\gamma }_{\mathit{i}}\left(x,t,U,U_x,V,\stackrel{.}{V}\right)$ at the boundary specified by IBND, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$.

11:   IRES – INTEGERInput/Output

On entry: set to $-1\text{ or }1$.

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

$\mathbf{IRES}=2$

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 $\mathbf{IRES}=3$, then D03PHF/D03PHA returns to the calling subroutine with the error indicator set to $\mathbf{IFAIL}=\mathbf{4}$.

Note: the following are additional parameters for specific use with D03PHA. Users of D03PHF 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 D03PHF/D03PHA. 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 D03PHF/D03PHA is called. Parameters denoted as Input must not be changed by this procedure.

7:     U(NEQN) – REAL (KIND=nag_wp) arrayInput/Output

On entry: the initial values of the dependent variables defined as follows:

• $\mathbf{U}\left(\mathbf{NPDE}\times \left(\mathit{j}-1\right)+\mathit{i}\right)$ contain $U_{\mathit{i}}\left(x_{\mathit{j}},t_0\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NPTS}$, and
• $\mathbf{U}\left(\mathbf{NPTS}\times \mathbf{NPDE}+\mathit{i}\right)$ contain $V_{\mathit{i}}\left(t_0\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

On exit: the computed solution $U_{\mathit{i}}\left(x_{\mathit{j}},t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NPTS}$, and $V_{\mathit{k}}\left(t\right)$, for $\mathit{k}=1,2,\dots ,\mathbf{NCODE}$, evaluated at $t=\mathbf{TS}$.

8:     NPTS – INTEGERInput

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

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

9:     X(NPTS) – REAL (KIND=nag_wp) arrayInput

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

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

10:   NCODE – INTEGERInput

On entry: the number of coupled ODE components.

Constraint: $\mathbf{NCODE}\ge 0$.

11:   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 ($\mathbf{NCODE}=0$), ODEDEF must be the dummy routine D03PCK/D53PCK for D03PHF (or D53PCK for D03PHA). D03PCK/D53PCK and D53PCK are included in the NAG Library.

The specification of ODEDEF for D03PHF 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 D03PHA 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 $\mathbf{NCODE}>0$, $\mathbf{V}\left(\mathit{i}\right)$ contains the value of the component $V_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

5:     VDOT(NCODE) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NCODE}>0$, $\mathbf{VDOT}\left(\mathit{i}\right)$ contains the value of component ${\stackrel{.}{V}}_{\mathit{i}}\left(t\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NCODE}$.

6:     NXI – INTEGERInput

On entry: the number of ODE/PDE coupling points.

7:     XI(NXI) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NXI}>0$, $\mathbf{XI}\left(\mathit{i}\right)$ contains the ODE/PDE coupling points, ${\xi }_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,\mathbf{NXI}$.

8:     UCP(NPDE,$*$) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NXI}>0$, $\mathbf{UCP}\left(\mathit{i},\mathit{j}\right)$ contains the value of $U_{\mathit{i}}\left(x,t\right)$ at the coupling point $x={\xi }_{\mathit{j}}$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NXI}$.

9:     UCPX(NPDE,$*$) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NXI}>0$, $\mathbf{UCPX}\left(\mathit{i},\mathit{j}\right)$ contains the value of $\frac{\partial U_{\mathit{i}}\left(x,t\right)}{\partial x}$ at the coupling point $x={\xi }_{\mathit{j}}$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NXI}$.

10:   RCP(NPDE,$*$) – REAL (KIND=nag_wp) arrayInput

On entry: $\mathbf{RCP}\left(\mathit{i},\mathit{j}\right)$ contains the value of the flux $R_{\mathit{i}}$ at the coupling point $x={\xi }_{\mathit{j}}$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NXI}$.

11:   UCPT(NPDE,$*$) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NXI}>0$, $\mathbf{UCPT}\left(\mathit{i},\mathit{j}\right)$ contains the value of $\frac{\partial U_{\mathit{i}}}{\partial t}$ at the coupling point $x={\xi }_{\mathit{j}}$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NXI}$.

12:   UCPTX(NPDE,$*$) – REAL (KIND=nag_wp) arrayInput

On entry: $\mathbf{UCPTX}\left(\mathit{i},\mathit{j}\right)$ contains the value of $\frac{{\partial }^2{U}_{\mathit{i}}}{\partial x\partial t}$ at the coupling point $x={\xi }_{\mathit{j}}$, for $\mathit{i}=1,2,\dots ,\mathbf{NPDE}$ and $\mathit{j}=1,2,\dots ,\mathbf{NXI}$.

13:   F(NCODE) – REAL (KIND=nag_wp) arrayOutput

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

$$F=G-A\stackrel{.}{V}-B\left(\begin{array}{c}{U}_t^{*}\\ U_{xt}^{*}\end{array}\right)\text{,}$$

(5)

or

$$F=-A\stackrel{.}{V}-B\left(\begin{array}{c}{U}_t^{*}\\ U_{xt}^{*}\end{array}\right)\text{.}$$

(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.

$\mathbf{IRES}=1$

Equation (5) must be used.

$\mathbf{IRES}=-1$

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:

$\mathbf{IRES}=2$

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 $\mathbf{IRES}=3$, then D03PHF/D03PHA returns to the calling subroutine with the error indicator set to $\mathbf{IFAIL}=\mathbf{4}$.

Note: the following are additional parameters for specific use with D03PHA. Users of D03PHF 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 D03PHF/D03PHA. 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 D03PHF/D03PHA is called. Parameters denoted as Input must not be changed by this procedure.

12:   NXI – INTEGERInput

On entry: the number of ODE/PDE coupling points.

Constraints:

• if $\mathbf{NCODE}=0$, $\mathbf{NXI}=0$;
• if $\mathbf{NCODE}>0$, $\mathbf{NXI}\ge 0$.

13:   XI(NXI) – REAL (KIND=nag_wp) arrayInput

On entry: if $\mathbf{NXI}>0$, $\mathbf{XI}\left(\mathit{i}\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NXI}$, must be set to the ODE/PDE coupling points.

Constraint: $\mathbf{X}\left(1\right)\le \mathbf{XI}\left(1\right)<\mathbf{XI}\left(2\right)<\cdots <\mathbf{XI}\left(\mathbf{NXI}\right)\le \mathbf{X}\left(\mathbf{NPTS}\right)$.

14:   NEQN – INTEGERInput

On entry: the number of ODEs in the time direction.

Constraint: $\mathbf{NEQN}=\mathbf{NPDE}\times \mathbf{NPTS}+\mathbf{NCODE}$.

15:   RTOL($*$) – REAL (KIND=nag_wp) arrayInput

Note: the dimension of the array RTOL must be at least $1$ if $\mathbf{ITOL}=1$ or $2$ and at least $\mathbf{NEQN}$ if $\mathbf{ITOL}=3$ or $4$.

On entry: the relative local error tolerance.

Constraint: $\mathbf{RTOL}\left(i\right)\ge 0.0$ for all relevant $i$.

16:   ATOL($*$) – REAL (KIND=nag_wp) arrayInput

Note: the dimension of the array ATOL must be at least $1$ if $\mathbf{ITOL}=1$ or $3$ and at least $\mathbf{NEQN}$ if $\mathbf{ITOL}=2$ or $4$.

On entry: the absolute local error tolerance.

Constraint: $\mathbf{ATOL}\left(i\right)\ge 0.0$ for all relevant $i$.

Note: corresponding elements of RTOL and ATOL cannot both be $0.0$.

17:   ITOL – INTEGERInput

On entry: a value to indicate the form of the local error test. ITOL indicates to D03PHF/D03PHA whether to interpret either or both of RTOL or ATOL as a vector or scalar. The error test to be satisfied is $‖e_i/w_i‖<1.0$, where $w_i$ is defined as follows:

ITOL	RTOL	ATOL	$w_i$
1	scalar	scalar	$\mathbf{RTOL}\left(1\right)\times \left|U_i\right|+\mathbf{ATOL}\left(1\right)$
2	scalar	vector	$\mathbf{RTOL}\left(1\right)\times \left|U_i\right|+\mathbf{ATOL}\left(i\right)$
3	vector	scalar	$\mathbf{RTOL}\left(i\right)\times \left|U_i\right|+\mathbf{ATOL}\left(1\right)$
4	vector	vector	$\mathbf{RTOL}\left(i\right)\times \left|U_i\right|+\mathbf{ATOL}\left(i\right)$

In the above, $e_{\mathit{i}}$ denotes the estimated local error for the $\mathit{i}$th component of the coupled PDE/ODE system in time, $\mathbf{U}\left(\mathit{i}\right)$, for $\mathit{i}=1,2,\dots ,\mathbf{NEQN}$.

The choice of norm used is defined by the parameter NORM.

Constraint: $1\le \mathbf{ITOL}\le 4$.

18:   NORM – CHARACTER(1)Input

On entry: the type of norm to be used.

$\mathbf{NORM}=\text{'M'}$

Maximum norm.

$\mathbf{NORM}=\text{'A'}$

Averaged $L_2$ norm.

If ${\mathbf{U}}_{\mathrm{norm}}$ denotes the norm of the vector U of length NEQN, then for the averaged $L_2$ norm

$${\mathbf{U}}_{\mathrm{norm}}=\sqrt{\frac{1}{\mathbf{NEQN}}\sum _{i=1}^{\mathbf{NEQN}}{\left(\mathbf{U}\left(i\right)/w_i\right)}^2}\text{,}$$

while for the maximum norm

$${\mathbf{U}}_{\mathrm{norm}}=\underset{i}{\max }\left|\mathbf{U}\left(i\right)/w_i\right|\text{.}$$

See the description of ITOL for the formulation of the weight vector $w$.

Constraint: $\mathbf{NORM}=\text{'M'}$ or $\text{'A'}$.

19:   LAOPT – CHARACTER(1)Input

On entry: the type of matrix algebra required.

$\mathbf{LAOPT}=\text{'F'}$

Full matrix methods to be used.

$\mathbf{LAOPT}=\text{'B'}$

Banded matrix methods to be used.

$\mathbf{LAOPT}=\text{'S'}$

Sparse matrix methods to be used.

Constraint: $\mathbf{LAOPT}=\text{'F'}$, $\text{'B'}$ or $\text{'S'}$.

Note: you are recommended to use the banded option when no coupled ODEs are present (i.e., $\mathbf{NCODE}=0$).

20:   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 $\mathbf{ALGOPT}\left(1\right)$ 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:

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

Selects the ODE integration method to be used. If $\mathbf{ALGOPT}\left(1\right)=1.0$, a BDF method is used and if $\mathbf{ALGOPT}\left(1\right)=2.0$, a Theta method is used. The default value is $\mathbf{ALGOPT}\left(1\right)=1.0$.

If $\mathbf{ALGOPT}\left(1\right)=2.0$, then $\mathbf{ALGOPT}\left(\mathit{i}\right)$, for $\mathit{i}=2,3,4$ are not used.

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

Specifies the maximum order of the BDF integration formula to be used. $\mathbf{ALGOPT}\left(2\right)$ may be $1.0$, $2.0$, $3.0$, $4.0$ or $5.0$. The default value is $\mathbf{ALGOPT}\left(2\right)=5.0$.

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

Specifies what method is to be used to solve the system of nonlinear equations arising on each step of the BDF method. If $\mathbf{ALGOPT}\left(3\right)=1.0$ a modified Newton iteration is used and if $\mathbf{ALGOPT}\left(3\right)=2.0$ 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 $\mathbf{ALGOPT}\left(3\right)=1.0$.

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

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 $P_{i,\mathit{j}}=0.0$, for $\mathit{j}=1,2,\dots ,\mathbf{NPDE}$, for some $i$ or when there is no ${\stackrel{.}{V}}_i\left(t\right)$ dependence in the coupled ODE system. If $\mathbf{ALGOPT}\left(4\right)=1.0$, then the Petzold test is used. If $\mathbf{ALGOPT}\left(4\right)=2.0$, then the Petzold test is not used. The default value is $\mathbf{ALGOPT}\left(4\right)=1.0$.

If $\mathbf{ALGOPT}\left(1\right)=1.0$, then $\mathbf{ALGOPT}\left(\mathit{i}\right)$, for $\mathit{i}=5,6,7$, are not used.

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

Specifies the value of Theta to be used in the Theta integration method. $0.51\le \mathbf{ALGOPT}\left(5\right)\le 0.99$. The default value is $\mathbf{ALGOPT}\left(5\right)=0.55$.

$\mathbf{ALGOPT}\left(6\right)$

Specifies what method is to be used to solve the system of nonlinear equations arising on each step of the Theta method. If $\mathbf{ALGOPT}\left(6\right)=1.0$, a modified Newton iteration is used and if $\mathbf{ALGOPT}\left(6\right)=2.0$, a functional iteration method is used. The default value is $\mathbf{ALGOPT}\left(6\right)=1.0$.

$\mathbf{ALGOPT}\left(7\right)$

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 $\mathbf{ALGOPT}\left(7\right)=1.0$, then switching is allowed and if $\mathbf{ALGOPT}\left(7\right)=2.0$, then switching is not allowed. The default value is $\mathbf{ALGOPT}\left(7\right)=1.0$.

$\mathbf{ALGOPT}\left(11\right)$

Specifies a point in the time direction, $t_{\mathrm{crit}}$, beyond which integration must not be attempted. The use of $t_{\mathrm{crit}}$ is described under the parameter ITASK. If $\mathbf{ALGOPT}\left(1\right)\ne 0.0$, a value of $0.0$ for $\mathbf{ALGOPT}\left(11\right)$, say, should be specified even if ITASK subsequently specifies that $t_{\mathrm{crit}}$ will not be used.

$\mathbf{ALGOPT}\left(12\right)$

Specifies the minimum absolute step size to be allowed in the time integration. If this option is not required, $\mathbf{ALGOPT}\left(12\right)$ should be set to $0.0$.

$\mathbf{ALGOPT}\left(13\right)$

Specifies the maximum absolute step size to be allowed in the time integration. If this option is not required, $\mathbf{ALGOPT}\left(13\right)$ should be set to $0.0$.

$\mathbf{ALGOPT}\left(14\right)$

Specifies the initial step size to be attempted by the integrator. If $\mathbf{ALGOPT}\left(14\right)=0.0$, then the initial step size is calculated internally.

$\mathbf{ALGOPT}\left(15\right)$

Specifies the maximum number of steps to be attempted by the integrator in any one call. If $\mathbf{ALGOPT}\left(15\right)=0.0$, then no limit is imposed.

$\mathbf{ALGOPT}\left(23\right)$

Specifies what method is to be used to solve the nonlinear equations at the initial point to initialize the values of $U$, $U_t$, $V$ and $\stackrel{.}{V}$. If $\mathbf{ALGOPT}\left(23\right)=1.0$, a modified Newton iteration is used and if $\mathbf{ALGOPT}\left(23\right)=2.0$, functional iteration is used. The default value is $\mathbf{ALGOPT}\left(23\right)=1.0$.

$\mathbf{ALGOPT}\left(29\right)$ and $\mathbf{ALGOPT}\left(30\right)$ are used only for the sparse matrix algebra option, $\mathbf{LAOPT}=\text{'S'}$.

$\mathbf{ALGOPT}\left(29\right)$

Governs the choice of pivots during the decomposition of the first Jacobian matrix. It should lie in the range $0.0<\mathbf{ALGOPT}\left(29\right)<1.0$, with smaller values biasing the algorithm towards maintaining sparsity at the expense of numerical stability. If $\mathbf{ALGOPT}\left(29\right)$ lies outside this range then the default value is used. If the routines regard the Jacobian matrix as numerically singular then increasing $\mathbf{ALGOPT}\left(29\right)$ towards $1.0$ may help, but at the cost of increased fill-in. The default value is $\mathbf{ALGOPT}\left(29\right)=0.1$.

$\mathbf{ALGOPT}\left(30\right)$

Is used as a relative pivot threshold during subsequent Jacobian decompositions (see $\mathbf{ALGOPT}\left(29\right)$) below which an internal error is invoked. If $\mathbf{ALGOPT}\left(30\right)$ 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 $\mathbf{ALGOPT}\left(29\right)$). The default value is $\mathbf{ALGOPT}\left(30\right)=0.0001$.

21:   RSAVE(LRSAVE) – 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.

22:   LRSAVE – INTEGERInput

On entry: the dimension of the array RSAVE as declared in the (sub)program from which D03PHF/D03PHA is called.

If $\mathbf{LAOPT}=\text{'F'}$, $\mathbf{LRSAVE}\ge \mathbf{NEQN}\times \mathbf{NEQN}+\mathbf{NEQN}+\mathit{nwkres}+\mathit{lenode}$.

If $\mathbf{LAOPT}=\text{'B'}$, $\mathbf{LRSAVE}\ge \left(3\times \mathit{mlu}+1\right)\times \mathbf{NEQN}+\mathit{nwkres}+\mathit{lenode}$.

If $\mathbf{LAOPT}=\text{'S'}$, $\mathbf{LRSAVE}\ge 4\times \mathbf{NEQN}+11\times \mathbf{NEQN}/2+1+\mathit{nwkres}+\mathit{lenode}$.

Where

	$\mathit{mlu}$ is the lower or upper half bandwidths such that $\mathit{mlu}=3\times \mathbf{NPDE}-1$, for PDE problems only (no coupled ODEs); or $\mathit{mlu}=\mathbf{NEQN}-1$, for coupled PDE/ODE problems.
	$\mathit{nwkres}=\left\{\begin{array}{ll}\mathbf{NPDE}\times \left(2\times \mathbf{NPTS}+6\times \mathbf{NXI}+3\times \mathbf{NPDE}+26\right)+\mathbf{NXI}+\mathbf{NCODE}+7\times \mathbf{NPTS}+2\text{,}& \text{when }\mathbf{NCODE}>0\text{ and }\mathbf{NXI}>0\text{;}\\ \mathbf{NPDE}\times \left(2\times \mathbf{NPTS}+3\times \mathbf{NPDE}+32\right)+\mathbf{NCODE}+7\times \mathbf{NPTS}+3\text{,}& \text{when }\mathbf{NCODE}>0\text{ and }\mathbf{NXI}=0\text{;}\\ \mathbf{NPDE}\times \left(2\times \mathbf{NPTS}+3\times \mathbf{NPDE}+32\right)+7\times \mathbf{NPTS}+4\text{,}& \text{when }\mathbf{NCODE}=0\text{.}\end{array}\right.$
	$\mathit{lenode}=\left\{\begin{array}{ll}\left(6+\mathrm{int}\left(\mathbf{ALGOPT}\left(2\right)\right)\right)\times \mathbf{NEQN}+50\text{,}& \text{when the BDF method is used; or}\\ 9\times \mathbf{NEQN}+50\text{,}& \text{when the Theta method is used.}\end{array}\right.$

Note: when $\mathbf{LAOPT}=\text{'S'}$, 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 $\mathbf{ITRACE}>0$ and the routine returns with $\mathbf{IFAIL}=\mathbf{15}$.

23:   ISAVE(LISAVE) – 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 back-substitution using the $LU$ decomposition of the Jacobian matrix.

24:   LISAVE – INTEGERInput

On entry: the dimension of the array ISAVE as declared in the (sub)program from which D03PHF/D03PHA is called. Its size depends on the type of matrix algebra selected:

• if $\mathbf{LAOPT}=\text{'F'}$, $\mathbf{LISAVE}\ge 24$;
• if $\mathbf{LAOPT}=\text{'B'}$, $\mathbf{LISAVE}\ge \mathbf{NEQN}+24$;
• if $\mathbf{LAOPT}=\text{'S'}$, $\mathbf{LISAVE}\ge 25\times \mathbf{NEQN}+24$.

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 $\mathbf{ITRACE}>0$ and the routine returns with $\mathbf{IFAIL}=\mathbf{15}$.

25:   ITASK – INTEGERInput

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

$\mathbf{ITASK}=1$

Normal computation of output values U at $t=\mathbf{TOUT}$.

$\mathbf{ITASK}=2$

One step and return.

$\mathbf{ITASK}=3$

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

$\mathbf{ITASK}=4$

Normal computation of output values U at $t=\mathbf{TOUT}$ but without overshooting $t=t_{\mathrm{crit}}$ where $t_{\mathrm{crit}}$ is described under the parameter ALGOPT.

$\mathbf{ITASK}=5$

Take one step in the time direction and return, without passing $t_{\mathrm{crit}}$, where $t_{\mathrm{crit}}$ is described under the parameter ALGOPT.

Constraint: $\mathbf{ITASK}=1$, $2$, $3$, $4$ or $5$.

26:   ITRACE – INTEGERInput

On entry: the level of trace information required from D03PHF/D03PHA and the underlying ODE solver. ITRACE may take the value $-1$, $0$, $1$, $2$ or $3$.

$\mathbf{ITRACE}=-1$

No output is generated.

$\mathbf{ITRACE}=0$

Only warning messages from the PDE solver are printed on the current error message unit (see X04AAF).

$\mathbf{ITRACE}>0$

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.

If $\mathbf{ITRACE}<-1$, then $-1$ is assumed and similarly if $\mathbf{ITRACE}>3$, then $3$ is assumed.

The advisory messages are given in greater detail as ITRACE increases. You are advised to set $\mathbf{ITRACE}=0$, unless you are experienced with sub-chapter D02M–N.

27:   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 parameters TOUT and IFAIL should be reset between calls to D03PHF/D03PHA.

Constraint: $\mathbf{IND}=0$ or $1$.

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

28:   IFAIL – INTEGERInput/Output

Note: for D03PHA, IFAIL does not occur in this position in the parameter list. See the additional parameters described below.

On entry: IFAIL must be set to $0$, $-1\text{ or }1$. 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 $-1\text{ or }1$ is recommended. If the output of error messages is undesirable, then the value $1$ is recommended. Otherwise, if you are not familiar with this parameter, 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).

Note: the following are additional parameters for specific use with D03PHA. Users of D03PHF therefore need not read the remainder of this description.

28:   IUSER($*$) – INTEGER arrayUser Workspace

29:   RUSER($*$) – REAL (KIND=nag_wp) arrayUser Workspace

IUSER and RUSER are not used by D03PHF/D03PHA, but are passed directly to PDEDEF, BNDARY and ODEDEF and may be used to pass information to these routines as an alternative to using COMMON global variables.

30:   CWSAV($10$) – CHARACTER(80) arrayCommunication Array

31:   LWSAV($100$) – LOGICAL arrayCommunication Array

32:   IWSAV($505$) – INTEGER arrayCommunication Array

33:   RWSAV($1100$) – REAL (KIND=nag_wp) arrayCommunication Array

34:   IFAIL – INTEGERInput/Output

Note: see the parameter description for IFAIL above.

6  Error Indicators and Warnings

$\mathbf{IFAIL}=1$

On entry,	$\mathbf{TOUT}-\mathbf{TS}$ is too small,
or	$\mathbf{ITASK}\ne 1$, $2$, $3$, $4$ or $5$,
or	$\mathbf{M}\ne 0$, $1$ or $2$,
or	at least one of the coupling points defined in array XI is outside the interval [$\mathbf{X}\left(1\right),\mathbf{X}\left(\mathbf{NPTS}\right)$],
or	$\mathbf{M}>0$ and $\mathbf{X}\left(1\right)<0.0$,
or	$\mathbf{NPTS}<3$,
or	$\mathbf{NPDE}<1$,
or	$\mathbf{NORM}\ne \text{'A'}$ or $\text{'M'}$,
or	$\mathbf{LAOPT}\ne \text{'F'}$, $\text{'B'}$ or $\text{'S'}$,
or	$\mathbf{ITOL}\ne 1$, $2$, $3$ or $4$,
or	$\mathbf{IND}\ne 0$ or $1$,
or	mesh points $\mathbf{X}\left(i\right)$ are badly ordered,
or	LRSAVE is too small,
or	LISAVE is too small,
or	NCODE and NXI are incorrectly defined,
or	$\mathbf{NEQN}\ne \mathbf{NPDE}\times \mathbf{NPTS}+\mathbf{NCODE}$,
or	either an element of RTOL or $\mathbf{ATOL}<0.0$,
or	all the elements of RTOL and ATOL are zero.

$\mathbf{IFAIL}=2$

The underlying ODE solver cannot make any further progress, with the values of ATOL and RTOL, across the integration range from the current point $t=\mathbf{TS}$. 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 error test failures on an attempted step, before completing the requested task, but the integration was successful as far as $t=\mathbf{TS}$. The problem may have a singularity, or the error requirement may be inappropriate.

$\mathbf{IFAIL}=4$

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 at least PDEDEF, BNDARY or ODEDEF, when the residual in the underlying ODE solver was being evaluated.

$\mathbf{IFAIL}=5$

In solving the ODE system, a singular Jacobian has been encountered. You should check your problem formulation.

$\mathbf{IFAIL}=6$

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 $t=\mathbf{TS}$.

$\mathbf{IFAIL}=7$

The values of ATOL and RTOL are so small that the routine is unable to start the integration in time.

$\mathbf{IFAIL}=8$

In one of PDEDEF, BNDARY or ODEDEF, 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 parameters 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 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 $\mathbf{ITASK}\ne 2$ or $5$.)

$\mathbf{IFAIL}=11$

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 $\mathbf{ALGOPT}\left(29\right)$ and $\mathbf{ALGOPT}\left(30\right)$ may be inappropriate.

$\mathbf{IFAIL}=12$

In solving the ODE system, the maximum number of steps specified in $\mathbf{ALGOPT}\left(15\right)$ have been taken.

$\mathbf{IFAIL}=13$

Some error weights $w_i$ became zero during the time integration (see the description of ITOL). Pure relative error control ($\mathbf{ATOL}\left(i\right)=0.0$) was requested on a variable (the $i$th) which has become zero. The integration was successful as far as $t=\mathbf{TS}$.

$\mathbf{IFAIL}=14$

The flux function $R_i$ was detected as depending on time derivatives, which is not permissible.

$\mathbf{IFAIL}=15$

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).

7  Accuracy

8  Further Comments

9  Example

9.1  Program Text

Program Text (d03phfe.f90)

Program Text (d03phae.f90)

9.2  Program Data

Program Data (d03phfe.d)

Program Data (d03phae.d)

9.3  Program Results

Program Results (d03phfe.r)

Program Results (d03phae.r)