NAG Library Routine Document

c05qbf  (sys_func_easy)

1

Purpose

2

Specification

3

Description

4

References

5

Arguments

1:     $\mathbf{fcn}$ – Subroutine, supplied by the user.External Procedure

fcn must return the values of the functions $f_i$ at a point $x$.

The specification of fcn is:

Fortran Interface

Subroutine fcn ( n, x, fvec, iuser, ruser, iflag) Integer, Intent (In) :: n Integer, Intent (Inout) :: iuser(*), iflag Real (Kind=nag_wp), Intent (In) :: x(n) Real (Kind=nag_wp), Intent (Inout) :: ruser(*) Real (Kind=nag_wp), Intent (Out) :: fvec(n)

C Header Interface

#include nagmk26.h void  fcn ( const Integer *n, const double x[], double fvec[], Integer iuser[], double ruser[], Integer *iflag)

1:     $\mathbf{n}$ – IntegerInput

On entry: $n$, the number of equations.

2:     $\mathbf{x}\left(\mathbf{n}\right)$ – Real (Kind=nag_wp) arrayInput

On entry: the components of the point $x$ at which the functions must be evaluated.

3:     $\mathbf{fvec}\left(\mathbf{n}\right)$ – Real (Kind=nag_wp) arrayOutput

On exit: the function values $f_i\left(x\right)$ (unless iflag is set to a negative value by fcn).

4:     $\mathbf{iuser}\left(*\right)$ – Integer arrayUser Workspace

5:     $\mathbf{ruser}\left(*\right)$ – Real (Kind=nag_wp) arrayUser Workspace

fcn is called with the arguments iuser and ruser as supplied to c05qbf. You should use the arrays iuser and ruser to supply information to fcn.

6:     $\mathbf{iflag}$ – IntegerInput/Output

On entry: $\mathbf{iflag}>0$.

On exit: in general, iflag should not be reset by fcn. If, however, you wish to terminate execution (perhaps because some illegal point x has been reached), iflag should be set to a negative integer.

fcn must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which c05qbf is called. Arguments denoted as Input must not be changed by this procedure.

Note: fcn should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by c05qbf. If your code inadvertently does return any NaNs or infinities, c05qbf is likely to produce unexpected results.

2:     $\mathbf{n}$ – IntegerInput

On entry: $n$, the number of equations.

Constraint: $\mathbf{n}>0$.

3:     $\mathbf{x}\left(\mathbf{n}\right)$ – Real (Kind=nag_wp) arrayInput/Output

On entry: an initial guess at the solution vector.

On exit: the final estimate of the solution vector.

4:     $\mathbf{fvec}\left(\mathbf{n}\right)$ – Real (Kind=nag_wp) arrayOutput

On exit: the function values at the final point returned in x.

5:     $\mathbf{xtol}$ – Real (Kind=nag_wp)Input

On entry: the accuracy in x to which the solution is required.

Suggested value: $\sqrt{\epsilon }$, where $\epsilon$ is the machine precision returned by x02ajf.

Constraint: $\mathbf{xtol}\ge 0.0$.

6:     $\mathbf{iuser}\left(*\right)$ – Integer arrayUser Workspace

7:     $\mathbf{ruser}\left(*\right)$ – Real (Kind=nag_wp) arrayUser Workspace

iuser and ruser are not used by c05qbf, but are passed directly to fcn and may be used to pass information to this routine.

8:     $\mathbf{ifail}$ – IntegerInput/Output

On entry: ifail must be set to $0$, $-1\text{ or }1$. If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation 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 argument, 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).

6

Error Indicators and Warnings

$\mathbf{ifail}=2$

There have been at least $200\times \left(\mathbf{n}+1\right)$ calls to fcn. Consider restarting the calculation from the point held in x.

$\mathbf{ifail}=3$

No further improvement in the solution is possible. xtol is too small: $\mathbf{xtol}=〈\mathit{\text{value}}〉$.

$\mathbf{ifail}=4$

The iteration is not making good progress. This failure exit may indicate that the system does not have a zero, or that the solution is very close to the origin (see Section 7). Otherwise, rerunning c05qbf from a different starting point may avoid the region of difficulty.

$\mathbf{ifail}=5$

iflag was set negative in fcn. $\mathbf{iflag}=〈\mathit{\text{value}}〉$.

$\mathbf{ifail}=11$

On entry, $\mathbf{n}=〈\mathit{\text{value}}〉$.
Constraint: $\mathbf{n}>0$.

$\mathbf{ifail}=12$

On entry, $\mathbf{xtol}=〈\mathit{\text{value}}〉$.
Constraint: $\mathbf{xtol}\ge 0.0$.

$\mathbf{ifail}=-99$

An unexpected error has been triggered by this routine. Please contact NAG.

See Section 3.9 in How to Use the NAG Library and its Documentation for further information.

$\mathbf{ifail}=-399$

Your licence key may have expired or may not have been installed correctly.

See Section 3.8 in How to Use the NAG Library and its Documentation for further information.

$\mathbf{ifail}=-999$

Dynamic memory allocation failed.

See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

7

Accuracy

8

Parallelism and Performance

9

Further Comments

10

Example

10.1

Program Text

Program Text (c05qbfe.f90)

10.2

Program Data

10.3

Program Results

Program Results (c05qbfe.r)