NAG Library Routine Document
C05AGF
1 Purpose
C05AGF locates a simple zero of a continuous function from a given starting value, using a binary search to locate an interval containing a zero of the function, then a combination of the methods of nonlinear interpolation, linear extrapolation and bisection to locate the zero precisely.
2 Specification
INTEGER 
IFAIL 
REAL (KIND=nag_wp) 
X, H, EPS, ETA, F, A, B 
EXTERNAL 
F 

3 Description
C05AGF attempts to locate an interval
$\left[a,b\right]$ containing a simple zero of the function
$f\left(x\right)$ by a binary search starting from the initial point
$x={\mathbf{X}}$ and using repeated calls to
C05AVF. If this search succeeds, then the zero is determined to a userspecified accuracy by a call to
C05AYF. The specifications of routines
C05AVF and
C05AYF should be consulted for details of the methods used.
The approximation
$x$ to the zero
$\alpha $ is determined so that at least one of the following criteria is satisfied:
(i) 
$\leftx\alpha \right\le {\mathbf{EPS}}$, 
(ii) 
$\leftf\left(x\right)\right\le {\mathbf{ETA}}$. 
4 References
Brent R P (1973) Algorithms for Minimization Without Derivatives Prentice–Hall
5 Parameters
 1: X – REAL (KIND=nag_wp)Input/Output
On entry: an initial approximation to the zero.
On exit: if
${\mathbf{IFAIL}}={\mathbf{0}}$ or
${\mathbf{4}}$,
X is the final approximation to the zero.
If
${\mathbf{IFAIL}}={\mathbf{3}}$,
X is likely to be a pole of
$f\left(x\right)$.
Otherwise,
X contains no useful information.
 2: H – REAL (KIND=nag_wp)Input
On entry: a step length for use in the binary search for an interval containing the zero. The maximum interval searched is $\left[{\mathbf{X}}256.0\times {\mathbf{H}},{\mathbf{X}}+256.0\times {\mathbf{H}}\right]$.
Constraint:
${\mathbf{H}}$ must be sufficiently large that ${\mathbf{X}}+{\mathbf{H}}\ne {\mathbf{X}}$ on the computer.
 3: EPS – REAL (KIND=nag_wp)Input
On entry: the termination tolerance on
$x$ (see
Section 3).
Constraint:
${\mathbf{EPS}}>0.0$.
 4: ETA – REAL (KIND=nag_wp)Input
On entry: a value such that if
$\leftf\left(x\right)\right\le {\mathbf{ETA}}$,
$x$ is accepted as the zero.
ETA may be specified as
$0.0$ (see
Section 7).
 5: F – REAL (KIND=nag_wp) FUNCTION, supplied by the user.External Procedure
F must evaluate the function
$f$ whose zero is to be determined.
The specification of
F is:
 1: XX – REAL (KIND=nag_wp)Input
On entry: the point at which the function must be evaluated.
F must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which C05AGF is called. Parameters denoted as
Input must
not be changed by this procedure.
 6: A – REAL (KIND=nag_wp)Output
 7: B – REAL (KIND=nag_wp)Output
On exit: the lower and upper bounds respectively of the interval resulting from the binary search. If the zero is determined exactly such that $f\left(x\right)=0.0$ or is determined so that $\leftf\left(x\right)\right\le {\mathbf{ETA}}$ at any stage in the calculation, then on exit ${\mathbf{A}}={\mathbf{B}}=x$.
 8: IFAIL – INTEGERInput/Output

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).
6 Error Indicators and Warnings
If on entry
${\mathbf{IFAIL}}={\mathbf{0}}$ or
${{\mathbf{1}}}$, explanatory error messages are output on the current error message unit (as defined by
X04AAF).
Errors or warnings detected by the routine:
 ${\mathbf{IFAIL}}=1$
On entry, either ${\mathbf{EPS}}\le 0.0$, or ${\mathbf{X}}+{\mathbf{H}}={\mathbf{X}}$ to machine accuracy (meaning that the search for an interval containing the zero cannot commence).
 ${\mathbf{IFAIL}}=2$
An interval containing the zero could not be found. Increasing
H and calling C05AGF again will increase the range searched for the zero. Decreasing
H and calling C05AGF again will refine the mesh used in the search for the zero.
 ${\mathbf{IFAIL}}=3$
A change in sign of
$f\left(x\right)$ has been determined as occurring near the point defined by the final value of
X. However, there is some evidence that this signchange corresponds to a pole of
$f\left(x\right)$.
 ${\mathbf{IFAIL}}=4$
Too much accuracy has been requested in the computation; that is, the zero has been located to relative accuracy at least
$\epsilon $, where
$\epsilon $ is the
machine precision, but the exit conditions described in
Section 3 are not satisfied. It is unsafe for C05AGF to continue beyond this point, but the final value of
X returned is an accurate approximation to the zero.
7 Accuracy
The levels of accuracy depend on the values of
EPS and
ETA. If full machine accuracy is required, they may be set very small, resulting in an exit with
${\mathbf{IFAIL}}={\mathbf{4}}$, although this may involve many more iterations than a lesser accuracy. You are recommended to set
${\mathbf{ETA}}=0.0$ and to use
EPS to control the accuracy, unless you have considerable knowledge of the size of
$f\left(x\right)$ for values of
$x$ near the zero.
The time taken by C05AGF depends primarily on the time spent evaluating
F (see
Section 5). The accuracy of the initial approximation
X and the value of
H will have a somewhat unpredictable effect on the timing.
If it is important to determine an interval of relative length less than
$2\times {\mathbf{EPS}}$ containing the zero, or if
F is expensive to evaluate and the number of calls to
F is to be restricted, then use of
C05AVF followed by
C05AZF is recommended. Use of this combination is also recommended when the structure of the problem to be solved does not permit a simple
F to be written: the reverse communication facilities of these routines are more flexible than the direct communication of
F required by C05AGF.
If the iteration terminates with successful exit and
${\mathbf{A}}={\mathbf{B}}={\mathbf{X}}$ there is no guarantee that the value returned in
X corresponds to a simple zero and you should check whether it does.
One way to check this is to compute the derivative of
$f$ at the point
X, preferably analytically, or, if this is not possible, numerically, perhaps by using a central difference estimate. If
${f}^{\prime}\left({\mathbf{X}}\right)=0.0$, then
X must correspond to a multiple zero of
$f$ rather than a simple zero.
9 Example
This example calculates an approximation to the zero of $x{e}^{x}$ using a tolerance of ${\mathbf{EPS}}=\text{1.0E\u22125}$ starting from ${\mathbf{X}}=1.0$ and using an initial search step ${\mathbf{H}}=0.1$.
9.1 Program Text
Program Text (c05agfe.f90)
9.2 Program Data
None.
9.3 Program Results
Program Results (c05agfe.r)