# NAG C Library Function Document

## 1Purpose

nag_inteq_volterra2 (d05bac) computes the solution of a nonlinear convolution Volterra integral equation of the second kind using a reducible linear multi-step method.

## 2Specification

 #include #include
void  nag_inteq_volterra2 (
 double (*ck)(double t, Nag_Comm *comm),
 double (*cg)(double s, double y, Nag_Comm *comm),
 double (*cf)(double t, Nag_Comm *comm),
Nag_ODEMethod method, Integer iorder, double alim, double tlim, double tol, Integer nmesh, double thresh, double work[], Integer lwk, double yn[], double errest[], Nag_Comm *comm, NagError *fail)

## 3Description

nag_inteq_volterra2 (d05bac) computes the numerical solution of the nonlinear convolution Volterra integral equation of the second kind
 $yt=ft+∫atkt-sgs,ysds, a≤t≤T.$ (1)
It is assumed that the functions involved in (1) are sufficiently smooth. The function uses a reducible linear multi-step formula selected by you to generate a family of quadrature rules. The reducible formulae available in nag_inteq_volterra2 (d05bac) are the Adams–Moulton formulae of orders $3$ to $6$, and the backward differentiation formulae (BDF) of orders $2$ to $5$. For more information about the behaviour and the construction of these rules we refer to Lubich (1983) and Wolkenfelt (1982).
The algorithm is based on computing the solution in a step-by-step fashion on a mesh of equispaced points. The initial step size which is given by $\left(T-a\right)/N$, $N$ being the number of points at which the solution is sought, is halved and another approximation to the solution is computed. This extrapolation procedure is repeated until successive approximations satisfy a user-specified error requirement.
The above methods require some starting values. For the Adams' formula of order greater than $3$ and the BDF of order greater than $2$ we employ an explicit Dormand–Prince–Shampine Runge–Kutta method (see Shampine (1986)). The above scheme avoids the calculation of the kernel, $k\left(t\right)$, on the negative real line.
Lubich Ch (1983) On the stability of linear multi-step methods for Volterra convolution equations IMA J. Numer. Anal. 3 439–465
Shampine L F (1986) Some practical Runge–Kutta formulas Math. Comput. 46(173) 135–150
Wolkenfelt P H M (1982) The construction of reducible quadrature rules for Volterra integral and integro-differential equations IMA J. Numer. Anal. 2 131–152

## 5Arguments

1:    $\mathbf{ck}$function, supplied by the userExternal Function
ck must evaluate the kernel $k\left(t\right)$ of the integral equation (1).
The specification of ck is:
 double ck (double t, Nag_Comm *comm)
1:    $\mathbf{t}$doubleInput
On entry: $t$, the value of the independent variable.
2:    $\mathbf{comm}$Nag_Comm *
Pointer to structure of type Nag_Comm; the following members are relevant to ck.
userdouble *
iuserInteger *
pPointer
The type Pointer will be void *. Before calling nag_inteq_volterra2 (d05bac) you may allocate memory and initialize these pointers with various quantities for use by ck when called from nag_inteq_volterra2 (d05bac) (see Section 3.3.1.1 in How to Use the NAG Library and its Documentation).
Note: ck should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by nag_inteq_volterra2 (d05bac). If your code inadvertently does return any NaNs or infinities, nag_inteq_volterra2 (d05bac) is likely to produce unexpected results.
2:    $\mathbf{cg}$function, supplied by the userExternal Function
cg must evaluate the function $g\left(s,y\left(s\right)\right)$ in (1).
The specification of cg is:
 double cg (double s, double y, Nag_Comm *comm)
1:    $\mathbf{s}$doubleInput
On entry: $s$, the value of the independent variable.
2:    $\mathbf{y}$doubleInput
On entry: the value of the solution $y$ at the point s.
3:    $\mathbf{comm}$Nag_Comm *
Pointer to structure of type Nag_Comm; the following members are relevant to cg.
userdouble *
iuserInteger *
pPointer
The type Pointer will be void *. Before calling nag_inteq_volterra2 (d05bac) you may allocate memory and initialize these pointers with various quantities for use by cg when called from nag_inteq_volterra2 (d05bac) (see Section 3.3.1.1 in How to Use the NAG Library and its Documentation).
Note: cg should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by nag_inteq_volterra2 (d05bac). If your code inadvertently does return any NaNs or infinities, nag_inteq_volterra2 (d05bac) is likely to produce unexpected results.
3:    $\mathbf{cf}$function, supplied by the userExternal Function
cf must evaluate the function $f\left(t\right)$ in (1).
The specification of cf is:
 double cf (double t, Nag_Comm *comm)
1:    $\mathbf{t}$doubleInput
On entry: $t$, the value of the independent variable.
2:    $\mathbf{comm}$Nag_Comm *
Pointer to structure of type Nag_Comm; the following members are relevant to cf.
userdouble *
iuserInteger *
pPointer
The type Pointer will be void *. Before calling nag_inteq_volterra2 (d05bac) you may allocate memory and initialize these pointers with various quantities for use by cf when called from nag_inteq_volterra2 (d05bac) (see Section 3.3.1.1 in How to Use the NAG Library and its Documentation).
Note: cf should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by nag_inteq_volterra2 (d05bac). If your code inadvertently does return any NaNs or infinities, nag_inteq_volterra2 (d05bac) is likely to produce unexpected results.
4:    $\mathbf{method}$Nag_ODEMethodInput
On entry: the type of method which you wish to employ.
${\mathbf{method}}=\mathrm{Nag_Adams}$
${\mathbf{method}}=\mathrm{Nag_BDF}$
For backward differentiation formulae.
Constraint: ${\mathbf{method}}=\mathrm{Nag_Adams}$ or $\mathrm{Nag_BDF}$.
5:    $\mathbf{iorder}$IntegerInput
On entry: the order of the method to be used.
Constraints:
• if ${\mathbf{method}}=\mathrm{Nag_Adams}$, $3\le {\mathbf{iorder}}\le 6$;
• if ${\mathbf{method}}=\mathrm{Nag_BDF}$, $2\le {\mathbf{iorder}}\le 5$.
6:    $\mathbf{alim}$doubleInput
On entry: $a$, the lower limit of the integration interval.
Constraint: ${\mathbf{alim}}\ge 0.0$.
7:    $\mathbf{tlim}$doubleInput
On entry: the final point of the integration interval, $T$.
Constraint: ${\mathbf{tlim}}>{\mathbf{alim}}$.
8:    $\mathbf{tol}$doubleInput
On entry: the relative accuracy required in the computed values of the solution.
Constraint: $\sqrt{\epsilon }\le {\mathbf{tol}}\le 1.0$, where $\epsilon$ is the machine precision.
9:    $\mathbf{nmesh}$IntegerInput
On entry: the number of equidistant points at which the solution is sought.
Constraints:
• if ${\mathbf{method}}=\mathrm{Nag_Adams}$, ${\mathbf{nmesh}}\ge {\mathbf{iorder}}-1$;
• if ${\mathbf{method}}=\mathrm{Nag_BDF}$, ${\mathbf{nmesh}}\ge {\mathbf{iorder}}$.
10:  $\mathbf{thresh}$doubleInput
On entry: the threshold value for use in the evaluation of the estimated relative errors. For two successive meshes the following condition must hold at each point of the coarser mesh
 $Y1-Y2 maxY1,Y2,thresh ≤tol,$
where ${Y}_{1}$ is the computed solution on the coarser mesh and ${Y}_{2}$ is the computed solution at the corresponding point in the finer mesh. If this condition is not satisfied then the step size is halved and the solution is recomputed.
Note: thresh can be used to effect a relative, absolute or mixed error test. If ${\mathbf{thresh}}=0.0$ then pure relative error is measured and, if the computed solution is small and ${\mathbf{thresh}}=1.0$, absolute error is measured.
11:  $\mathbf{work}\left[{\mathbf{lwk}}\right]$doubleOutput
12:  $\mathbf{lwk}$IntegerInput
On entry: the dimension of the array work.
Constraint: ${\mathbf{lwk}}\ge 10×{\mathbf{nmesh}}+6$.
Note: the above value of lwk is sufficient for nag_inteq_volterra2 (d05bac) to perform only one extrapolation on the initial mesh as defined by nmesh. In general much more workspace is required and in the case when a large step size is supplied (i.e., nmesh is small), you must provide a considerably larger workspace.
On exit: if ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NW_OUT_OF_WORKSPACE, ${\mathbf{work}}\left[0\right]$ contains the size of lwk required for the algorithm to proceed further.
13:  $\mathbf{yn}\left[{\mathbf{nmesh}}\right]$doubleOutput
On exit: ${\mathbf{yn}}\left[\mathit{i}-1\right]$ contains the most recent approximation of the true solution $y\left(t\right)$ at the specified point $t={\mathbf{alim}}+\mathit{i}×H$, for $\mathit{i}=1,2,\dots ,{\mathbf{nmesh}}$, where $H=\left({\mathbf{tlim}}-{\mathbf{alim}}\right)/{\mathbf{nmesh}}$.
14:  $\mathbf{errest}\left[{\mathbf{nmesh}}\right]$doubleOutput
On exit: ${\mathbf{errest}}\left[\mathit{i}-1\right]$ contains the most recent approximation of the relative error in the computed solution at the point $t={\mathbf{alim}}+\mathit{i}×H$, for $\mathit{i}=1,2,\dots ,{\mathbf{nmesh}}$, where $H=\left({\mathbf{tlim}}-{\mathbf{alim}}\right)/{\mathbf{nmesh}}$.
15:  $\mathbf{comm}$Nag_Comm *
The NAG communication argument (see Section 3.3.1.1 in How to Use the NAG Library and its Documentation).
16:  $\mathbf{fail}$NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

## 6Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 2.3.1.2 in How to Use the NAG Library and its Documentation for further information.
On entry, argument $〈\mathit{\text{value}}〉$ had an illegal value.
NE_CONVERGENCE
The solution is not converging. See Section 9.
NE_ENUM_INT
On entry, ${\mathbf{method}}=\mathrm{Nag_Adams}$ and ${\mathbf{iorder}}=2$.
Constraint: if ${\mathbf{method}}=\mathrm{Nag_Adams}$, $3\le {\mathbf{iorder}}\le 6$.
On entry, ${\mathbf{method}}=\mathrm{Nag_BDF}$ and ${\mathbf{iorder}}=6$.
Constraint: if ${\mathbf{method}}=\mathrm{Nag_BDF}$, $2\le {\mathbf{iorder}}\le 5$.
NE_ENUM_INT_2
On entry, ${\mathbf{method}}=\mathrm{Nag_Adams}$, ${\mathbf{iorder}}=〈\mathit{\text{value}}〉$ and ${\mathbf{nmesh}}=〈\mathit{\text{value}}〉$.
Constraint: if ${\mathbf{method}}=\mathrm{Nag_Adams}$, ${\mathbf{nmesh}}\ge {\mathbf{iorder}}-1$.
On entry, ${\mathbf{method}}=\mathrm{Nag_BDF}$, ${\mathbf{iorder}}=〈\mathit{\text{value}}〉$ and ${\mathbf{nmesh}}=〈\mathit{\text{value}}〉$.
Constraint: if ${\mathbf{method}}=\mathrm{Nag_BDF}$, ${\mathbf{nmesh}}\ge {\mathbf{iorder}}$.
NE_INT
On entry, ${\mathbf{iorder}}=〈\mathit{\text{value}}〉$.
Constraint: $2\le {\mathbf{iorder}}\le 6$.
On entry, ${\mathbf{lwk}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{lwk}}\ge 10×{\mathbf{nmesh}}+6$; that is, $〈\mathit{\text{value}}〉$.
NE_INTERNAL_ERROR
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
See Section 2.7.6 in How to Use the NAG Library and its Documentation for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 2.7.5 in How to Use the NAG Library and its Documentation for further information.
NE_REAL
On entry, ${\mathbf{alim}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{alim}}\ge 0.0$.
On entry, ${\mathbf{tol}}=〈\mathit{\text{value}}〉$.
Constraint: .
NE_REAL_2
On entry, ${\mathbf{alim}}=〈\mathit{\text{value}}〉$ and ${\mathbf{tlim}}=〈\mathit{\text{value}}〉$.
Constraint: ${\mathbf{tlim}}>{\mathbf{alim}}$.
NW_OUT_OF_WORKSPACE
The workspace which has been supplied is too small for the required accuracy. The number of extrapolations, so far, is $〈\mathit{\text{value}}〉$. If you require one more extrapolation extend the size of workspace to: ${\mathbf{lwk}}=〈\mathit{\text{value}}〉$.

## 7Accuracy

The accuracy depends on tol, the theoretical behaviour of the solution of the integral equation, the interval of integration and on the method being used. It can be controlled by varying tol and thresh; you are recommended to choose a smaller value for tol, the larger the value of iorder.
You are warned not to supply a very small tol, because the required accuracy may never be achieved. This will usually force an error exit with ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NW_OUT_OF_WORKSPACE.
In general, the higher the order of the method, the faster the required accuracy is achieved with less workspace. For non-stiff problems (see Section 9) you are recommended to use the Adams' method (${\mathbf{method}}=\mathrm{Nag_Adams}$) of order greater than $4$ (${\mathbf{iorder}}>4$).

## 8Parallelism and Performance

nag_inteq_volterra2 (d05bac) makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the x06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

When solving (1), the solution of a nonlinear equation of the form
 $Yn-αgtn,Yn-Ψn=0,$ (2)
is required, where ${\Psi }_{n}$ and $\alpha$ are constants. nag_inteq_volterra2 (d05bac) calls nag_interval_zero_cont_func (c05avc) to find an interval for the zero of this equation followed by nag_zero_cont_func_brent_rcomm (c05azc) to find its zero.
There is an initial phase of the algorithm where the solution is computed only for the first few points of the mesh. The exact number of these points depends on iorder and method. The step size is halved until the accuracy requirements are satisfied on these points and only then the solution on the whole mesh is computed. During this initial phase, if lwk is too small, nag_inteq_volterra2 (d05bac) will exit with ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NW_OUT_OF_WORKSPACE.
In the case ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NE_CONVERGENCE or NW_OUT_OF_WORKSPACE, you may be dealing with a ‘stiff’ equation; an equation where the Lipschitz constant $L$ of the function $g\left(t,y\right)$ in (1) with respect to its second argument is large, viz,
 $gt,u-gt,v≤Lu-v.$ (3)
In this case, if a BDF method (${\mathbf{method}}=\mathrm{Nag_BDF}$) has been used, you are recommended to choose a smaller step size by increasing the value of nmesh, or provide a larger workspace. But, if an Adams' method (${\mathbf{method}}=\mathrm{Nag_Adams}$) has been selected, you are recommended to switch to a BDF method instead.
In the case ${\mathbf{fail}}\mathbf{.}\mathbf{code}=$ NW_OUT_OF_WORKSPACE, then if ${\mathbf{fail}}\mathbf{.}\mathbf{errnum}=6$, the specified accuracy has not been attained but yn and errest contain the most recent approximation to the computed solution and the corresponding error estimate. In this case, the error message informs you of the number of extrapolations performed and the size of lwk required for the algorithm to proceed further. The latter quantity will also be available in ${\mathbf{work}}\left[0\right]$.

## 10Example

Consider the following integral equation
 $yt=e-t+∫0te-t-sys+e-ysds, 0≤t≤20$ (4)
with the solution $y\left(t\right)=\mathrm{ln}\left(t+e\right)$. In this example, the Adams' method of order $6$ is used to solve this equation with ${\mathbf{tol}}=\text{1.e−4}$.

### 10.1Program Text

Program Text (d05bace.c)

None.

### 10.3Program Results

Program Results (d05bace.r)