# NAG FL Interfacec05qbf (sys_​func_​easy)

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## 1Purpose

c05qbf is an easy-to-use routine that finds a solution of a system of nonlinear equations by a modification of the Powell hybrid method.

## 2Specification

Fortran Interface
 Subroutine c05qbf ( fcn, n, x, fvec, xtol,
 Integer, Intent (In) :: n Integer, Intent (Inout) :: iuser(*), ifail Real (Kind=nag_wp), Intent (In) :: xtol Real (Kind=nag_wp), Intent (Inout) :: x(n), ruser(*) Real (Kind=nag_wp), Intent (Out) :: fvec(n) External :: fcn
#include <nag.h>
 void c05qbf_ (void (NAG_CALL *fcn)(const Integer *n, const double x[], double fvec[], Integer iuser[], double ruser[], Integer *iflag),const Integer *n, double x[], double fvec[], const double *xtol, Integer iuser[], double ruser[], Integer *ifail)
The routine may be called by the names c05qbf or nagf_roots_sys_func_easy.

## 3Description

The system of equations is defined as:
 $fi (x1,x2,…,xn) = 0 , ​ i= 1, 2, …, n .$
c05qbf is based on the MINPACK routine HYBRD1 (see Moré et al. (1980)). It chooses the correction at each step as a convex combination of the Newton and scaled gradient directions. The Jacobian is updated by the rank-1 method of Broyden. At the starting point, the Jacobian is approximated by forward differences, but these are not used again until the rank-1 method fails to produce satisfactory progress. For more details see Powell (1970).
Moré J J, Garbow B S and Hillstrom K E (1980) User guide for MINPACK-1 Technical Report ANL-80-74 Argonne National Laboratory
Powell M J D (1970) A hybrid method for nonlinear algebraic equations Numerical Methods for Nonlinear Algebraic Equations (ed P Rabinowitz) Gordon and Breach

## 5Arguments

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,
 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)
 void fcn (const Integer *n, const double x[], double fvec[], Integer iuser[], double ruser[], Integer *iflag)
1: $\mathbf{n}$Integer Input
On entry: $n$, the number of equations.
2: $\mathbf{x}\left({\mathbf{n}}\right)$Real (Kind=nag_wp) array Input
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) array Output
On exit: the function values ${f}_{i}\left(x\right)$.
4: $\mathbf{iuser}\left(*\right)$Integer array User Workspace
5: $\mathbf{ruser}\left(*\right)$Real (Kind=nag_wp) array User 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: $\mathbf{iflag}$Integer Input/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: $\mathbf{n}$Integer Input
On entry: $n$, the number of equations.
Constraint: ${\mathbf{n}}>0$.
3: $\mathbf{x}\left({\mathbf{n}}\right)$Real (Kind=nag_wp) array Input/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) array Output
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 array User Workspace
7: $\mathbf{ruser}\left(*\right)$Real (Kind=nag_wp) array User 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: $\mathbf{ifail}$Integer Input/Output
On entry: ifail must be set to $0$, $-1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $-1$ means that an error message is printed while a value of $1$ means that it is not.
If halting is not appropriate, the value $-1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. When the value $-\mathbf{1}$ 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).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=2$
There have been at least $200×\left({\mathbf{n}}+1\right)$ 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.
${\mathbf{ifail}}=5$
Termination requested in fcn.
${\mathbf{ifail}}=11$
On entry, ${\mathbf{n}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{n}}>0$.
${\mathbf{ifail}}=12$
On entry, ${\mathbf{xtol}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{xtol}}\ge 0.0$.
${\mathbf{ifail}}=-99$
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

## 7Accuracy

If $\stackrel{^}{x}$ is the true solution, c05qbf tries to ensure that
 $‖x-x^‖2 ≤ xtol × ‖x^‖2 .$
If this condition is satisfied with ${\mathbf{xtol}}={10}^{-k}$, then the larger components of $x$ have $k$ significant decimal digits. There is a danger that the smaller components of $x$ may have large relative errors, but the fast rate of convergence of c05qbf usually obviates this possibility.
If xtol is less than machine precision and the above test is satisfied with the machine precision in place of xtol, then the routine exits with ${\mathbf{ifail}}={\mathbf{3}}$.
Note:  this convergence test is based purely on relative error, and may not indicate convergence if the solution is very close to the origin.
The convergence test assumes that the functions are reasonably well behaved. If this condition is not satisfied, then c05qbf may incorrectly indicate convergence. The validity of the answer can be checked, for example, by rerunning c05qbf with a lower value for xtol.

## 8Parallelism and Performance

c05qbf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
c05qbf 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 routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

Local workspace arrays of fixed lengths are allocated internally by c05qbf. The total size of these arrays amounts to $n×\left(3×n+13\right)/2$ real elements.
The time required by c05qbf to solve a given problem depends on $n$, the behaviour of the functions, the accuracy requested and the starting point. The number of arithmetic operations executed by c05qbf to process each evaluation of the functions is approximately $11.5×{n}^{2}$. The timing of c05qbf is strongly influenced by the time spent evaluating the functions.
Ideally the problem should be scaled so that, at the solution, the function values are of comparable magnitude.

## 10Example

This example determines the values ${x}_{1},\dots ,{x}_{9}$ which satisfy the tridiagonal equations:
 $(3-2x1)x1-2x2 = −1, -xi-1+(3-2xi)xi-2xi+1 = −1, i=2,3,…,8 -x8+(3-2x9)x9 = −1.$

### 10.1Program Text

Program Text (c05qbfe.f90)

None.

### 10.3Program Results

Program Results (c05qbfe.r)