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Chapter Contents
Chapter Introduction
NAG Toolbox

# NAG Toolbox: nag_pde_1d_parab_remesh_fd (d03pp)

## Purpose

nag_pde_1d_parab_remesh_fd (d03pp) 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).

## Syntax

[ts, u, x, rsave, isave, ind, user, cwsav, lwsav, iwsav, rwsav, ifail] = d03pp(npde, m, ts, tout, pdedef, bndary, uvinit, u, x, ncode, odedef, xi, rtol, atol, itol, norm_p, laopt, algopt, remesh, xfix, nrmesh, dxmesh, trmesh, ipminf, monitf, rsave, isave, itask, itrace, ind, cwsav, lwsav, iwsav, rwsav, 'npts', npts, 'nxi', nxi, 'neqn', neqn, 'nxfix', nxfix, 'xratio', xratio, 'con', con, 'user', user)
[ts, u, x, rsave, isave, ind, user, cwsav, lwsav, iwsav, rwsav, ifail] = nag_pde_1d_parab_remesh_fd(npde, m, ts, tout, pdedef, bndary, uvinit, u, x, ncode, odedef, xi, rtol, atol, itol, norm_p, laopt, algopt, remesh, xfix, nrmesh, dxmesh, trmesh, ipminf, monitf, rsave, isave, itask, itrace, ind, cwsav, lwsav, iwsav, rwsav, 'npts', npts, 'nxi', nxi, 'neqn', neqn, 'nxfix', nxfix, 'xratio', xratio, 'con', con, 'user', user)
Note: the interface to this routine has changed since earlier releases of the toolbox:
 At Mark 22: lrsave and lisave were removed from the interface

## Description

nag_pde_1d_parab_remesh_fd (d03pp) integrates the system of parabolic-elliptic equations and coupled ODEs
 $∑j=1npdePi,j ∂Uj ∂t +Qi=x-m ∂∂x xmRi, i=1,2,…,npde , a≤x≤b,t≥t0,$ (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), ${P}_{i,j}$ and ${R}_{i}$ depend on $x$, $t$, $U$, ${U}_{x}$, and $V$; ${Q}_{i}$ depends on $x$, $t$, $U$, ${U}_{x}$, $V$ and linearly on $\stackrel{.}{V}$. The vector $U$ is the set of PDE solution values
 $Ux,t=U1x,t,…,Unpdex,tT,$
and the vector ${U}_{x}$ is the partial derivative with respect to $x$. The vector $V$ is the set of ODE solution values
 $Vt=V1t,…,VncodetT,$
and $\stackrel{.}{V}$ denotes its derivative with respect to time.
In (2), $\xi$ represents a vector of ${n}_{\xi }$ 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. ${U}^{*}$, ${U}_{x}^{*}$, ${R}^{*}$, ${U}_{t}^{*}$ and ${U}_{xt}^{*}$ are the functions $U$, ${U}_{x}$, $R$, ${U}_{t}$ and ${U}_{xt}$ evaluated at these coupling points. Each ${F}_{i}$ 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 $F={\left[{F}_{1},\dots ,{F}_{{\mathbf{ncode}}}\right]}^{\mathrm{T}}$, $G$ is a vector of length ncode, $A$ is an ncode by ncode matrix, $B$ is an ncode by $\left({n}_{\xi }×{\mathbf{npde}}\right)$ matrix and the entries in $G$, $A$ and $B$ may depend on $t$, $\xi$, ${U}^{*}$, ${U}_{x}^{*}$ 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 Arguments for the specification of odedef.)
The integration in time is from ${t}_{0}$ to ${t}_{\mathrm{out}}$, over the space interval $a\le x\le b$, where $a={x}_{1}$ and $b={x}_{{\mathbf{npts}}}$ are the leftmost and rightmost points of a mesh ${x}_{1},{x}_{2},\dots ,{x}_{{\mathbf{npts}}}$ 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 ${P}_{i,j}$, ${Q}_{i}$ and ${R}_{i}$ must be specified in pdedef.
The initial $\left(t={t}_{0}\right)$ values of the functions $U\left(x,t\right)$ and $V\left(t\right)$ must be specified in uvinit. Note that uvinit will be called again following any initial remeshing, and so $U\left(x,{t}_{0}\right)$ should be specified for all values of $x$ in the interval $a\le x\le b$, and not just the initial mesh points.
The functions ${R}_{i}$ 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 ${\gamma }_{i}$ may depend linearly on $\stackrel{.}{V}$.
The problem is subject to the following restrictions:
 (i) In (1), ${\stackrel{.}{V}}_{\mathit{j}}\left(t\right)$, for $\mathit{j}=1,2,\dots ,{\mathbf{ncode}}$, may only appear linearly in the functions ${Q}_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$, with a similar restriction for $\gamma$; (ii) ${P}_{i,j}$ and the flux ${R}_{i}$ must not depend on any time derivatives; (iii) ${t}_{0}<{t}_{\mathrm{out}}$, so that integration is in the forward direction; (iv) The evaluation of the terms ${P}_{\mathit{i},j}$, ${Q}_{\mathit{i}}$ and ${R}_{\mathit{i}}$ is done approximately at the mid-points of the mesh ${\mathbf{x}}\left(\mathit{i}\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{npts}}$, 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 ${P}_{i,j}$ must be nonzero so that there is a time derivative present in the PDE problem; (vi) If $m>0$ and ${x}_{1}=0.0$, 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 ${\beta }_{i}=1.0$ and ${\gamma }_{i}=0.0$. See also Further Comments.
The algebraic-differential equation system which is defined by the functions ${F}_{i}$ must be specified in odedef. You must also specify the coupling points $\xi$ in the array xi.
The parabolic equations are approximated by a system of ODEs in time for the values of ${U}_{i}$ 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 ${\mathbf{npde}}×{\mathbf{npts}}+{\mathbf{ncode}}$ 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.

## 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

## Parameters

### Compulsory Input Parameters

1:     $\mathrm{npde}$int64int32nag_int scalar
The number of PDEs to be solved.
Constraint: ${\mathbf{npde}}\ge 1$.
2:     $\mathrm{m}$int64int32nag_int scalar
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:     $\mathrm{ts}$ – double scalar
The initial value of the independent variable $t$.
Constraint: ${\mathbf{ts}}<{\mathbf{tout}}$.
4:     $\mathrm{tout}$ – double scalar
The final value of $t$ to which the integration is to be carried out.
5:     $\mathrm{pdedef}$ – function handle or string containing name of m-file
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 nag_pde_1d_parab_remesh_fd (d03pp).
[p, q, r, ires, user] = pdedef(npde, t, x, u, ux, ncode, v, vdot, ires, user)

Input Parameters

1:     $\mathrm{npde}$int64int32nag_int scalar
The number of PDEs in the system.
2:     $\mathrm{t}$ – double scalar
The current value of the independent variable $t$.
3:     $\mathrm{x}$ – double scalar
The current value of the space variable $x$.
4:     $\mathrm{u}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{u}}\left(\mathit{i}\right)$ contains the value of the component ${U}_{\mathit{i}}\left(x,t\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
5:     $\mathrm{ux}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{ux}}\left(\mathit{i}\right)$ 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:     $\mathrm{ncode}$int64int32nag_int scalar
The number of coupled ODEs in the system.
7:     $\mathrm{v}\left({\mathbf{ncode}}\right)$ – double array
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:     $\mathrm{vdot}\left({\mathbf{ncode}}\right)$ – double array
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:     $\mathrm{ires}$int64int32nag_int scalar
Set to $-1\text{​ or ​}1$.
10:   $\mathrm{user}$ – Any MATLAB object
pdedef is called from nag_pde_1d_parab_remesh_fd (d03pp) with the object supplied to nag_pde_1d_parab_remesh_fd (d03pp).

Output Parameters

1:     $\mathrm{p}\left({\mathbf{npde}},{\mathbf{npde}}\right)$ – double array
${\mathbf{p}}\left(\mathit{i},\mathit{j}\right)$ 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 $\mathit{j}=1,2,\dots ,{\mathbf{npde}}$.
2:     $\mathrm{q}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{q}}\left(\mathit{i}\right)$ must be set to the value of ${Q}_{\mathit{i}}\left(x,t,U,{U}_{x},V,\stackrel{.}{V}\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
3:     $\mathrm{r}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{r}}\left(\mathit{i}\right)$ must be set to the value of ${R}_{\mathit{i}}\left(x,t,U,{U}_{x},V\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
4:     $\mathrm{ires}$int64int32nag_int scalar
Should usually remain unchanged. However, you may set ires to force the integration function 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 nag_pde_1d_parab_remesh_fd (d03pp) returns to the calling function with the error indicator set to ${\mathbf{ifail}}={\mathbf{4}}$.
5:     $\mathrm{user}$ – Any MATLAB object
6:     $\mathrm{bndary}$ – function handle or string containing name of m-file
bndary must evaluate the functions ${\beta }_{i}$ and ${\gamma }_{i}$ which describe the boundary conditions, as given in (4).
[beta, gamma, ires, user] = bndary(npde, t, u, ux, ncode, v, vdot, ibnd, ires, user)

Input Parameters

1:     $\mathrm{npde}$int64int32nag_int scalar
The number of PDEs in the system.
2:     $\mathrm{t}$ – double scalar
The current value of the independent variable $t$.
3:     $\mathrm{u}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{u}}\left(\mathit{i}\right)$ 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:     $\mathrm{ux}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{ux}}\left(\mathit{i}\right)$ 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:     $\mathrm{ncode}$int64int32nag_int scalar
The number of coupled ODEs in the system.
6:     $\mathrm{v}\left({\mathbf{ncode}}\right)$ – double array
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:     $\mathrm{vdot}\left({\mathbf{ncode}}\right)$ – double array
${\mathbf{vdot}}\left(\mathit{i}\right)$ 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 ${\gamma }_{\mathit{j}}$, for $\mathit{j}=1,2,\dots ,{\mathbf{npde}}$.
8:     $\mathrm{ibnd}$int64int32nag_int scalar
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:     $\mathrm{ires}$int64int32nag_int scalar
Set to $-1\text{​ or ​}1$.
10:   $\mathrm{user}$ – Any MATLAB object
bndary is called from nag_pde_1d_parab_remesh_fd (d03pp) with the object supplied to nag_pde_1d_parab_remesh_fd (d03pp).

Output Parameters

1:     $\mathrm{beta}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{beta}}\left(\mathit{i}\right)$ must be set to the value of ${\beta }_{\mathit{i}}\left(x,t\right)$ at the boundary specified by ibnd, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
2:     $\mathrm{gamma}\left({\mathbf{npde}}\right)$ – double array
${\mathbf{gamma}}\left(\mathit{i}\right)$ 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 $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$.
3:     $\mathrm{ires}$int64int32nag_int scalar
Should usually remain unchanged. However, you may set ires to force the integration function 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 nag_pde_1d_parab_remesh_fd (d03pp) returns to the calling function with the error indicator set to ${\mathbf{ifail}}={\mathbf{4}}$.
4:     $\mathrm{user}$ – Any MATLAB object
7:     $\mathrm{uvinit}$ – function handle or string containing name of m-file
uvinit must supply the initial $\left(t={t}_{0}\right)$ values of $U\left(x,t\right)$ and $V\left(t\right)$ for all values of $x$ in the interval $a\le x\le b$.
[u, v, user] = uvinit(npde, npts, nxi, x, xi, ncode, user)

Input Parameters

1:     $\mathrm{npde}$int64int32nag_int scalar
The number of PDEs in the system.
2:     $\mathrm{npts}$int64int32nag_int scalar
The number of mesh points in the interval $\left[a,b\right]$.
3:     $\mathrm{nxi}$int64int32nag_int scalar
The number of ODE/PDE coupling points.
4:     $\mathrm{x}\left({\mathbf{npts}}\right)$ – double array
The current mesh. ${\mathbf{x}}\left(\mathit{i}\right)$ contains the value of ${x}_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,{\mathbf{npts}}$.
5:     $\mathrm{xi}\left({\mathbf{nxi}}\right)$ – double array
If ${\mathbf{nxi}}>0$, ${\mathbf{xi}}\left(\mathit{i}\right)$ contains the value of the ODE/PDE coupling point, ${\xi }_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,{\mathbf{nxi}}$.
6:     $\mathrm{ncode}$int64int32nag_int scalar
The number of coupled ODEs in the system.
7:     $\mathrm{user}$ – Any MATLAB object
uvinit is called from nag_pde_1d_parab_remesh_fd (d03pp) with the object supplied to nag_pde_1d_parab_remesh_fd (d03pp).

Output Parameters

1:     $\mathrm{u}\left({\mathbf{npde}},{\mathbf{npts}}\right)$ – double array
If ${\mathbf{nxi}}>0$, ${\mathbf{u}}\left(\mathit{i},\mathit{j}\right)$ contains the value of the component ${U}_{\mathit{i}}\left({x}_{\mathit{j}},{t}_{0}\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$ and $\mathit{j}=1,2,\dots ,{\mathbf{npts}}$.
2:     $\mathrm{v}\left({\mathbf{ncode}}\right)$ – double array
${\mathbf{v}}\left(\mathit{i}\right)$ contains the value of component ${V}_{\mathit{i}}\left({t}_{0}\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{ncode}}$.
3:     $\mathrm{user}$ – Any MATLAB object
8:     $\mathrm{u}\left({\mathbf{neqn}}\right)$ – double array
If ${\mathbf{ind}}=1$, the value of u must be unchanged from the previous call.
9:     $\mathrm{x}\left({\mathbf{npts}}\right)$ – double array
The initial 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:   $\mathrm{ncode}$int64int32nag_int scalar
The number of coupled ODE in the system.
Constraint: ${\mathbf{ncode}}\ge 0$.
11:   $\mathrm{odedef}$ – function handle or string containing name of m-file
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 string nag_pde_1d_parab_remesh_fd_dummy_odedef (d53pck).
[f, ires, user] = odedef(npde, t, ncode, v, vdot, nxi, xi, ucp, ucpx, rcp, ucpt, ucptx, ires, user)

Input Parameters

1:     $\mathrm{npde}$int64int32nag_int scalar
The number of PDEs in the system.
2:     $\mathrm{t}$ – double scalar
The current value of the independent variable $t$.
3:     $\mathrm{ncode}$int64int32nag_int scalar
The number of coupled ODEs in the system.
4:     $\mathrm{v}\left({\mathbf{ncode}}\right)$ – double array
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:     $\mathrm{vdot}\left({\mathbf{ncode}}\right)$ – double array
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:     $\mathrm{nxi}$int64int32nag_int scalar
The number of ODE/PDE coupling points.
7:     $\mathrm{xi}\left({\mathbf{nxi}}\right)$ – double array
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:     $\mathrm{ucp}\left({\mathbf{npde}},:\right)$ – double array
The second dimension of the array ucp must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nxi}}\right)$.
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:     $\mathrm{ucpx}\left({\mathbf{npde}},:\right)$ – double array
The second dimension of the array ucpx must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nxi}}\right)$.
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:   $\mathrm{rcp}\left({\mathbf{npde}},:\right)$ – double array
The second dimension of the array rcp must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nxi}}\right)$.
${\mathbf{rcp}}\left(\mathit{i},\mathit{j}\right)$ 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:   $\mathrm{ucpt}\left({\mathbf{npde}},:\right)$ – double array
The second dimension of the array ucpt must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nxi}}\right)$.
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:   $\mathrm{ucptx}\left({\mathbf{npde}},:\right)$ – double array
The second dimension of the array ucptx must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nxi}}\right)$.
${\mathbf{ucptx}}\left(\mathit{i},\mathit{j}\right)$ 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:   $\mathrm{ires}$int64int32nag_int scalar
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.
14:   $\mathrm{user}$ – Any MATLAB object
odedef is called from nag_pde_1d_parab_remesh_fd (d03pp) with the object supplied to nag_pde_1d_parab_remesh_fd (d03pp).

Output Parameters

1:     $\mathrm{f}\left({\mathbf{ncode}}\right)$ – double array
${\mathbf{f}}\left(\mathit{i}\right)$ must contain the $\mathit{i}$th component of $F$, for $\mathit{i}=1,2,\dots ,{\mathbf{ncode}}$, where $F$ is defined as
 $F=G-AV.-B Ut* Uxt* ,$ (5)
or
 $F=-AV.-B Ut* Uxt* .$ (6)
The definition of $F$ is determined by the input value of ires.
2:     $\mathrm{ires}$int64int32nag_int scalar
Should usually remain unchanged. However, you may reset ires to force the integration function 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 nag_pde_1d_parab_remesh_fd (d03pp) returns to the calling function with the error indicator set to ${\mathbf{ifail}}={\mathbf{4}}$.
3:     $\mathrm{user}$ – Any MATLAB object
12:   $\mathrm{xi}\left({\mathbf{nxi}}\right)$ – double array
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: ${\mathbf{x}}\left(1\right)\le {\mathbf{xi}}\left(1\right)<{\mathbf{xi}}\left(2\right)<\cdots <{\mathbf{xi}}\left({\mathbf{nxi}}\right)\le {\mathbf{x}}\left({\mathbf{npts}}\right)$.
13:   $\mathrm{rtol}\left(:\right)$ – double array
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 $4$
The relative local error tolerance.
Constraint: ${\mathbf{rtol}}\left(i\right)\ge 0.0$ for all relevant $i$.
14:   $\mathrm{atol}\left(:\right)$ – double array
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 $4$
The absolute local error tolerance.
Constraints:
• ${\mathbf{atol}}\left(i\right)\ge 0.0$ for all relevant $i$;
• Corresponding elements of atol and rtol cannot both be $0.0$.
15:   $\mathrm{itol}$int64int32nag_int scalar
A value to indicate the form of the local error test. itol indicates to nag_pde_1d_parab_remesh_fd (d03pp) 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 ${\mathbf{rtol}}\left(1\right)×\left|{U}_{i}\right|+{\mathbf{atol}}\left(1\right)$ 2 scalar vector ${\mathbf{rtol}}\left(1\right)×\left|{U}_{i}\right|+{\mathbf{atol}}\left(i\right)$ 3 vector scalar ${\mathbf{rtol}}\left(i\right)×\left|{U}_{i}\right|+{\mathbf{atol}}\left(1\right)$ 4 vector vector ${\mathbf{rtol}}\left(i\right)×\left|{U}_{i}\right|+{\mathbf{atol}}\left(i\right)$
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 argument norm_p.
Constraint: $1\le {\mathbf{itol}}\le 4$.
16:   $\mathrm{norm_p}$ – string (length ≥ 1)
The type of norm to be used.
${\mathbf{norm_p}}=\text{'M'}$
Maximum norm.
${\mathbf{norm_p}}=\text{'A'}$
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
 $unorm=1neqn∑i=1neqnui/wi2,$
while for the maximum norm
 $u norm = maxi ui / wi .$
See the description of itol for the formulation of the weight vector $w$.
Constraint: ${\mathbf{norm_p}}=\text{'M'}$ or $\text{'A'}$.
17:   $\mathrm{laopt}$ – string (length ≥ 1)
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., ${\mathbf{ncode}}=0$).
18:   $\mathrm{algopt}\left(30\right)$ – double array
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 argument 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 functions 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 ${\mathbf{algopt}}\left(30\right)=0.0001$.
19:   $\mathrm{remesh}$ – logical scalar
Indicates whether or not spatial remeshing should be performed.
${\mathbf{remesh}}=\mathit{true}$
Indicates that spatial remeshing should be performed as specified.
${\mathbf{remesh}}=\mathit{false}$
Indicates that spatial remeshing should be suppressed.
Note:  remesh should not be changed between consecutive calls to nag_pde_1d_parab_remesh_fd (d03pp). Remeshing can be switched off or on at specified times by using appropriate values for the arguments nrmesh and trmesh at each call.
20:   $\mathrm{xfix}\left(:\right)$ – double array
The dimension of the array xfix must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,{\mathbf{nxfix}}\right)$
${\mathbf{xfix}}\left(\mathit{i}\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{nxfix}}$, must contain the value of the $x$ coordinate at the $\mathit{i}$th fixed mesh point.
Constraints:
• ${\mathbf{xfix}}\left(\mathit{i}\right)<{\mathbf{xfix}}\left(\mathit{i}+1\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{nxfix}}-1$;
• each fixed mesh point must coincide with a user-supplied initial mesh point, that is ${\mathbf{xfix}}\left(\mathit{i}\right)={\mathbf{x}}\left(\mathit{j}\right)$ for some $j$, $2\le j\le {\mathbf{npts}}-1$.
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.
21:   $\mathrm{nrmesh}$int64int32nag_int scalar
Specifies the spatial remeshing frequency and criteria for the calculation and adoption of a new mesh.
${\mathbf{nrmesh}}<0$
Indicates that a new mesh is adopted according to the argument dxmesh. The mesh is tested every $\left|{\mathbf{nrmesh}}\right|$ timesteps.
${\mathbf{nrmesh}}=0$
Indicates that remeshing should take place just once at the end of the first time step reached when $t>{\mathbf{trmesh}}$.
${\mathbf{nrmesh}}>0$
Indicates that remeshing will take place every nrmesh time steps, with no testing using dxmesh.
Note: nrmesh may be changed between consecutive calls to nag_pde_1d_parab_remesh_fd (d03pp) to give greater flexibility over the times of remeshing.
22:   $\mathrm{dxmesh}$ – double scalar
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 $\left|{\mathbf{nrmesh}}\right|$ time steps, but is adopted only if
 $xinew>xi old +dxmesh×xi+1 old -xi old$
or
 $xinew
dxmesh thus imposes a lower limit on the difference between one mesh and the next.
Constraint: ${\mathbf{dxmesh}}\ge 0.0$.
23:   $\mathrm{trmesh}$ – double scalar
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 nag_pde_1d_parab_remesh_fd (d03pp) to force remeshing at several specified times.
24:   $\mathrm{ipminf}$int64int32nag_int scalar
The level of trace information regarding the adaptive remeshing. Details are directed to the current advisory message unit (see nag_file_set_unit_advisory (x04ab)).
${\mathbf{ipminf}}=0$
No trace information.
${\mathbf{ipminf}}=1$
Brief summary of mesh characteristics.
${\mathbf{ipminf}}=2$
More detailed information, including old and new mesh points, mesh sizes and monitor function values.
Constraint: ${\mathbf{ipminf}}=0$, $1$ or $2$.
25:   $\mathrm{monitf}$ – function handle or string containing name of m-file
monitf must supply and evaluate a remesh monitor function to indicate the solution behaviour of interest.
If you specify ${\mathbf{remesh}}=\mathit{false}$, i.e., no remeshing, then monitf will not be called and the string nag_pde_1d_parab_remesh_fd_dummy_monitf (d53pcl) may be used for monitf.
[fmon, user] = monitf(t, npts, npde, x, u, r, user)

Input Parameters

1:     $\mathrm{t}$ – double scalar
The current value of the independent variable $t$.
2:     $\mathrm{npts}$int64int32nag_int scalar
The number of mesh points in the interval $\left[a,b\right]$.
3:     $\mathrm{npde}$int64int32nag_int scalar
The number of PDEs in the system.
4:     $\mathrm{x}\left({\mathbf{npts}}\right)$ – double array
The current mesh. ${\mathbf{x}}\left(\mathit{i}\right)$ contains the value of ${x}_{\mathit{i}}$, for $\mathit{i}=1,2,\dots ,{\mathbf{npts}}$.
5:     $\mathrm{u}\left({\mathbf{npde}},{\mathbf{npts}}\right)$ – double array
${\mathbf{u}}\left(\mathit{i},\mathit{j}\right)$ contains the value of ${U}_{\mathit{i}}\left(x,t\right)$ at $x={\mathbf{x}}\left(\mathit{j}\right)$ and time $t$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$ and $\mathit{j}=1,2,\dots ,{\mathbf{npts}}$.
6:     $\mathrm{r}\left({\mathbf{npde}},{\mathbf{npts}}\right)$ – double array
${\mathbf{r}}\left(\mathit{i},\mathit{j}\right)$ contains the value of ${R}_{\mathit{i}}\left(x,t,U,{U}_{x},V\right)$ at $x={\mathbf{x}}\left(\mathit{j}\right)$ and time $t$, for $\mathit{i}=1,2,\dots ,{\mathbf{npde}}$ and $\mathit{j}=1,2,\dots ,{\mathbf{npts}}$.
7:     $\mathrm{user}$ – Any MATLAB object
monitf is called from nag_pde_1d_parab_remesh_fd (d03pp) with the object supplied to nag_pde_1d_parab_remesh_fd (d03pp).

Output Parameters

1:     $\mathrm{fmon}\left({\mathbf{npts}}\right)$ – double array
${\mathbf{fmon}}\left(i\right)$ must contain the value of the monitor function ${F}^{\mathrm{mon}}\left(x\right)$ at mesh point $x={\mathbf{x}}\left(i\right)$.
2:     $\mathrm{user}$ – Any MATLAB object
26:   $\mathrm{rsave}\left(\mathit{lrsave}\right)$ – double 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 function because it contains required information about the iteration.
27:   $\mathrm{isave}\left(\mathit{lisave}\right)$int64int32nag_int 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 function because it contains required information about the iteration required for subsequent calls. 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 ODE method last used in the time integration.
${\mathbf{isave}}\left(5\right)$
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.
28:   $\mathrm{itask}$int64int32nag_int scalar
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 argument 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 argument algopt.
Constraint: ${\mathbf{itask}}=1$, $2$, $3$, $4$ or $5$.
29:   $\mathrm{itrace}$int64int32nag_int scalar
The level of trace information required from nag_pde_1d_parab_remesh_fd (d03pp) and the underlying ODE solver:
${\mathbf{itrace}}\le -1$
No output is generated.
${\mathbf{itrace}}=0$
Only warning messages from the PDE solver are printed on the current error message unit (see nag_file_set_unit_error (x04aa)).
${\mathbf{itrace}}=1$
Output from the underlying ODE solver is printed on the current advisory message unit (see nag_file_set_unit_advisory (x04ab)). This output contains details of Jacobian entries, the nonlinear iteration and the time integration during the computation of the ODE system.
${\mathbf{itrace}}=2$
Output from the underlying ODE solver is similar to that produced when ${\mathbf{itrace}}=1$, except that the advisory messages are given in greater detail.
${\mathbf{itrace}}\ge 3$
Output from the underlying ODE solver is similar to that produced when ${\mathbf{itrace}}=2$, except that the advisory messages are given in greater detail.
You are advised to set ${\mathbf{itrace}}=0$, unless you are experienced with Sub-chapter D02M–N.
30:   $\mathrm{ind}$int64int32nag_int scalar
Must be set to $0$ or $1$.
${\mathbf{ind}}=0$
Starts or restarts the integration in time.
${\mathbf{ind}}=1$
Continues the integration after an earlier exit from the function. In this case, only the arguments tout and ifail and the remeshing arguments nrmesh, dxmesh, trmesh, xratio and con may be reset between calls to nag_pde_1d_parab_remesh_fd (d03pp).
Constraint: $0\le {\mathbf{ind}}\le 1$.
31:   $\mathrm{cwsav}\left(10\right)$ – cell array of strings
32:   $\mathrm{lwsav}\left(100\right)$ – logical array
33:   $\mathrm{iwsav}\left(505\right)$int64int32nag_int array
34:   $\mathrm{rwsav}\left(1100\right)$ – double array

### Optional Input Parameters

1:     $\mathrm{npts}$int64int32nag_int scalar
Default: the dimension of the array x.
The number of mesh points in the interval $\left[a,b\right]$.
Constraint: ${\mathbf{npts}}\ge 3$.
2:     $\mathrm{nxi}$int64int32nag_int scalar
Default: the dimension of the array xi.
The number of ODE/PDE coupling points.
Constraints:
• if ${\mathbf{ncode}}=0$, ${\mathbf{nxi}}=0$;
• if ${\mathbf{ncode}}>0$, ${\mathbf{nxi}}\ge 0$.
3:     $\mathrm{neqn}$int64int32nag_int scalar
Default: the dimension of the array u.
The number of ODEs in the time direction.
Constraint: ${\mathbf{neqn}}={\mathbf{npde}}×{\mathbf{npts}}+{\mathbf{ncode}}$.
4:     $\mathrm{nxfix}$int64int32nag_int scalar
Default: the dimension of the array xfix.
The number of fixed mesh points.
Constraint: $0\le {\mathbf{nxfix}}\le {\mathbf{npts}}-2$.
Note: the end points ${\mathbf{x}}\left(1\right)$ and ${\mathbf{x}}\left({\mathbf{npts}}\right)$ are fixed automatically and hence should not be specified as fixed points.
5:     $\mathrm{xratio}$ – double scalar
Default: $1.5$
An input bound on the adjacent mesh ratio (greater than $1.0$ and typically in the range $1.5$ to $3.0$). The remeshing functions will attempt to ensure that
 $xi-xi-1/xratio
Constraint: ${\mathbf{xratio}}>1.0$.
6:     $\mathrm{con}$ – double scalar
Default: $2.0/\left({\mathbf{npts}}-1\right)$
An input bound on the sub-integral of the monitor function ${F}^{\mathrm{mon}}\left(x\right)$ over each space step. The remeshing functions will attempt to ensure that
 $∫xixi+1Fmonxdx≤con∫x1xnptsFmonxdx,$
(see Furzeland (1984)). con gives you more control over the mesh distribution e.g., decreasing con allows more clustering. A typical value is $2/\left({\mathbf{npts}}-1\right)$, 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.
Constraint: $0.1/\left({\mathbf{npts}}-1\right)\le {\mathbf{con}}\le 10.0/\left({\mathbf{npts}}-1\right)$.
7:     $\mathrm{user}$ – Any MATLAB object
user is not used by nag_pde_1d_parab_remesh_fd (d03pp), but is passed to pdedef, bndary, uvinit, odedef and monitf. Note that for large objects it may be more efficient to use a global variable which is accessible from the m-files than to use user.

### Output Parameters

1:     $\mathrm{ts}$ – double scalar
The value of $t$ corresponding to the solution values in u. Normally ${\mathbf{ts}}={\mathbf{tout}}$.
2:     $\mathrm{u}\left({\mathbf{neqn}}\right)$ – double array
${\mathbf{u}}\left({\mathbf{npde}}×\left(\mathit{j}-1\right)+\mathit{i}\right)$ contains 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 ${\mathbf{u}}\left({\mathbf{npts}}×{\mathbf{npde}}+\mathit{k}\right)$ contains ${V}_{\mathit{k}}\left(t\right)$, for $\mathit{k}=1,2,\dots ,{\mathbf{ncode}}$, evaluated at $t={\mathbf{ts}}$.
3:     $\mathrm{x}\left({\mathbf{npts}}\right)$ – double array
The final values of the mesh points.
4:     $\mathrm{rsave}\left(\mathit{lrsave}\right)$ – double array
If ${\mathbf{ind}}=1$, rsave must be unchanged from the previous call to the function because it contains required information about the iteration.
5:     $\mathrm{isave}\left(\mathit{lisave}\right)$int64int32nag_int array
If ${\mathbf{ind}}=1$, isave must be unchanged from the previous call to the function because it contains required information about the iteration required for subsequent calls. 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 ODE method last used in the time integration.
${\mathbf{isave}}\left(5\right)$
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.
6:     $\mathrm{ind}$int64int32nag_int scalar
${\mathbf{ind}}=1$.
7:     $\mathrm{user}$ – Any MATLAB object
8:     $\mathrm{cwsav}\left(10\right)$ – cell array of strings
9:     $\mathrm{lwsav}\left(100\right)$ – logical array
10:   $\mathrm{iwsav}\left(505\right)$int64int32nag_int array
11:   $\mathrm{rwsav}\left(1100\right)$ – double array
12:   $\mathrm{ifail}$int64int32nag_int scalar
${\mathbf{ifail}}={\mathbf{0}}$ unless the function detects an error (see Error Indicators and Warnings).

## Error Indicators and Warnings

Errors or warnings detected by the function:

Cases prefixed with W are classified as warnings and do not generate an error of type NAG:error_n. See nag_issue_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 $\left[{\mathbf{x}}\left(1\right),{\mathbf{x}}\left({\mathbf{npts}}\right)\right]$, or ${\mathbf{m}}>0$ and ${\mathbf{x}}\left(1\right)<0.0$, or ${\mathbf{npts}}<3$, or ${\mathbf{npde}}<1$, or ${\mathbf{norm_p}}\ne \text{'A'}$ 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 an element of rtol or ${\mathbf{atol}}<0.0$, or corresponding elements of rtol and atol are both $0.0$, or ${\mathbf{neqn}}\ne {\mathbf{npde}}×{\mathbf{npts}}+{\mathbf{ncode}}$, or nxfix not in the range $0$ to ${\mathbf{npts}}-2$, or fixed mesh point(s) do not coincide with any of the user-supplied mesh points, or ${\mathbf{dxmesh}}<0.0$, or ${\mathbf{ipminf}}\ne 0$, $1$ or $2$, or ${\mathbf{xratio}}\le 1.0$, or con not in the range $0.1/\left({\mathbf{npts}}-1\right)$ to $10/\left({\mathbf{npts}}-1\right)$.
W  ${\mathbf{ifail}}=2$
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 $t={\mathbf{ts}}$.
W  ${\mathbf{ifail}}=3$
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 function 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.
W  ${\mathbf{ifail}}=6$
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 function 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.
${\mathbf{ifail}}=9$ (nag_ode_ivp_stiff_imp_revcom (d02nn))
A serious error has occurred in an internal call to the specified function. Check the problem specification and all arguments and array dimensions. Setting ${\mathbf{itrace}}=1$ may provide more information. If the problem persists, contact NAG.
W  ${\mathbf{ifail}}=10$
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.
W  ${\mathbf{ifail}}=13$
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).
${\mathbf{ifail}}=16$
remesh has been changed between calls to nag_pde_1d_parab_remesh_fd (d03pp), which is not permissible.
${\mathbf{ifail}}=17$
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 ${\mathbf{ipminf}}=1$ may provide more information.
${\mathbf{ifail}}=-99$
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.

## Accuracy

nag_pde_1d_parab_remesh_fd (d03pp) 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 arguments, atol and rtol.

The argument 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 function nag_pde_1d_parab_remesh_keller (d03pr).
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.

## Example

This example uses Burgers Equation, a common test problem for remeshing algorithms, given by
 $∂U ∂t =-U ∂U ∂x +E ∂2U ∂x2 ,$
for $x\in \left[0,1\right]$ and $t\in \left[0,1\right]$, 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
 $A = 50Ex- 0.5+ 4.95t, B = 250Ex- 0.5+ 0.75t, C = 500Ex- 0.375.$
```function d03pp_example

fprintf('d03pp example results\n\n');

% Initialize input arguments
npde   = int64(1);
m      = int64(0);
ts     = 0;
tout   = 0.2;
npts   = 61;
u      = zeros(npts, 1);
dx     = 1/(npts-1);
x      = [0:dx:1];
ncode  = int64(0);
xi     = [];
rtol   = [5e-05];
atol   = [5e-05];
itol   = int64(1);
normt  = 'A';
laopt  = 'F';
algopt = zeros(30, 1);
remesh = true;
xfix   = [];
nrmesh = int64(3);
dxmesh = 0.5;
trmesh = 0;
ipminf = int64(0);
rsave  = zeros(5018, 1);
isave  = zeros(25, 1, 'int64');
itrace = int64(0);
ind    = int64(0);
cwsav  = {''; ''; ''; ''; ''; ''; ''; ''; ''; ''};
lwsav  = false(100, 1);
iwsav  = zeros(505, 1, 'int64');
rwsav =  zeros(1100, 1);

% Integrate from t=0 to t=1 in steps of 0.1
usol = zeros(11,npts);
xsol = zeros(11,npts);
tsol = zeros(11,npts);
tout = [0.01,0.02,0.04,0.06,0.08,0.1,0.2,0.3,0.4,0.7,1.0];
for j = 1:11
[ts, u, x, rsave, isave, ind, user, cwsav, lwsav, iwsav, rwsav, ifail] = ...
d03pp( ...
npde, m, ts, tout(j), @pdedef, @bndary, @uvinit, ...
u, x, ncode, @odedef, xi, rtol, atol, itol, normt, laopt, ...
algopt, remesh, xfix, nrmesh, dxmesh, trmesh, ipminf, @monitf, ...
rsave, isave, itask, itrace, ind, cwsav, lwsav, iwsav, rwsav);

xsol(j,:) = x;
tsol(j,:) = ts;
usol(j,:) = u;

end

fprintf(' Number of time steps           = %6d\n',isave(1));
fprintf(' Number of function evaluations = %6d\n',isave(2));
fprintf(' Number of Jacobian evaluations = %6d\n',isave(3));
fprintf(' Number of iterations           = %6d\n',isave(5));

fig1=figure;
mesh(xsol,tsol,usol);
title('Burger''s Equation solution using moving mesh');
xlabel('x');
ylabel('t');
zlabel('u(x,t)');
view(22,24);

function [p, q, r, ires, user] = pdedef(npde, t, x, u, ux, ncode, v, vdot, ...
ires, user)
p = zeros(npde, npde);
q = zeros(npde, 1);
r = zeros(npde, 1);

p(1,1) = 1.0d0;
r(1) = 0.005*ux(1);
q(1) = u(1)*ux(1);

function [beta, gamma, ires, user] = bndary(npde, t, u, ux, ncode, v, ...
vdot, ibnd, ires, user)
beta = zeros(npde, 1);
gamma = zeros(npde, 1);

if (ibnd == 0)
ue = exact(t,1,[0]);
else
ue = exact(t,1,[1]);
end
gamma(1) = u(1) - ue(1);

function [u, v, user] = uvinit(npde, npts, nxi, x, xi, ncode, user)
u = zeros(npde, npts);
v = zeros(ncode, 1);

u(1,1:npts) = exact(0,npts,x);

function [f, ires, user] = odedef(npde, t, ncode, v, vdot, nxi, xi, ...
ucp, ucpx, rcp, ucpt, ucptx, ires, user)
f = zeros(ncode, 1);

function [fmon, user] = monitf(t, npts, npde, x, u, r, user)
fmon = zeros(npts, 1);

drdx = (r(2)-r(1))/(x(2)-x(1));
fmon(1) = abs(drdx);
for i = 2:npts-1
% second derivative ..
drdx  = 2*(r(1,i+1)-r(1,i))/(x(i+1)-x(i-1));
fmon(i) = abs(drdx);
end
fmon(npts) = fmon(npts-1);

function ue = exact(t,n,x);
e = 0.005;
ue = zeros(n,1);
for i = 1:n
a = (x(i)-0.25-0.75*t)/(4*e);
b = (0.9*x(i)-0.325-0.495*t)/(2*e);
if (a>0 && a>b)
a = exp(-a);
c = (0.8*x(i)-0.4-0.24*t)/(4*e);
c = exp(c);
ue(i) = (0.5+0.1*c+a)/(1+c+a);
elseif (b>0 && b>=a)
b = exp(-b);
c = (-0.8*x(i)+0.4+0.24*t)/(4*e);
c = exp(c);
ue(i) = (0.1+c/2+b)/(1+c+b);
else
a = exp(a);
b = exp(b);
ue(i) = (1+a/2+0.1*b)/(1+a+b);
end
end
```
```d03pp example results

Number of time steps           =    199
Number of function evaluations =   4737
Number of Jacobian evaluations =     69
Number of iterations           =    505
```