NAG Library Routine Document
D01AQF calculates an approximation to the Hilbert transform of a function
for user-specified values of
|SUBROUTINE D01AQF (
||G, A, B, C, EPSABS, EPSREL, RESULT, ABSERR, W, LW, IW, LIW, IFAIL)
||LW, IW(LIW), LIW, IFAIL
||G, A, B, C, EPSABS, EPSREL, RESULT, ABSERR, W(LW)
D01AQF is based on the QUADPACK routine QAWC (see Piessens et al. (1983)
) and integrates a function of the form
, where the weight function
is that of the Hilbert transform. (If
the integral has to be interpreted in the sense of a Cauchy principal value.) It is an adaptive routine which employs a ‘global’ acceptance criterion (as defined by Malcolm and Simpson (1976)
). Special care is taken to ensure that
is never the end point of a sub-interval (see Piessens et al. (1976)
). On each sub-interval
modified Clenshaw–Curtis integration of orders
is performed if
. Otherwise the Gauss
-point and Kronrod
-point rules are used. The local error estimation is described by
Piessens et al. (1983)
Malcolm M A and Simpson R B (1976) Local versus global strategies for adaptive quadrature ACM Trans. Math. Software 1 129–146
Piessens R, de Doncker–Kapenga E, Überhuber C and Kahaner D (1983) QUADPACK, A Subroutine Package for Automatic Integration Springer–Verlag
Piessens R, van Roy–Branders M and Mertens I (1976) The automatic evaluation of Cauchy principal value integrals Angew. Inf. 18 31–35
- 1: G – REAL (KIND=nag_wp) FUNCTION, supplied by the user.External Procedure
must return the value of the function
at a given point X
The specification of G
- 1: X – REAL (KIND=nag_wp)Input
On entry: the point at which the function must be evaluated.
must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which D01AQF is called. Parameters denoted as Input
be changed by this procedure.
- 2: A – REAL (KIND=nag_wp)Input
On entry: , the lower limit of integration.
- 3: B – REAL (KIND=nag_wp)Input
On entry: , the upper limit of integration. It is not necessary that .
- 4: C – REAL (KIND=nag_wp)Input
On entry: the parameter in the weight function.
must not equal A
- 5: EPSABS – REAL (KIND=nag_wp)Input
: the absolute accuracy required. If EPSABS
is negative, the absolute value is used. See Section 7
- 6: EPSREL – REAL (KIND=nag_wp)Input
: the relative accuracy required. If EPSREL
is negative, the absolute value is used. See Section 7
- 7: RESULT – REAL (KIND=nag_wp)Output
On exit: the approximation to the integral .
- 8: ABSERR – REAL (KIND=nag_wp)Output
On exit: an estimate of the modulus of the absolute error, which should be an upper bound for .
- 9: W(LW) – REAL (KIND=nag_wp) arrayOutput
: details of the computation see Section 8
for more information.
- 10: LW – INTEGERInput
: the dimension of the array W
as declared in the (sub)program from which D01AQF is called. The value of LW
(together with that of LIW
) imposes a bound on the number of sub-intervals into which the interval of integration may be divided by the routine. The number of sub-intervals cannot exceed
. The more difficult the integrand, the larger LW
to is adequate for most problems.
- 11: IW(LIW) – INTEGER arrayOutput
On exit: contains the actual number of sub-intervals used. The rest of the array is used as workspace.
- 12: LIW – INTEGERInput
: the dimension of the array IW
as declared in the (sub)program from which D01AQF is called. The number of sub-intervals into which the interval of integration may be divided cannot exceed LIW
- 13: IFAIL – INTEGERInput/Output
must be set to
. 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
is recommended. If the output of error messages is undesirable, then the value
is recommended. Otherwise, because for this routine the values of the output parameters may be useful even if
on exit, the recommended value is
. When the value is used it is essential to test the value of IFAIL on exit.
unless the routine detects an error or a warning has been flagged (see Section 6
6 Error Indicators and Warnings
If on entry
, explanatory error messages are output on the current error message unit (as defined by X04AAF
Note: D01AQF may return useful information for one or more of the following detected errors or warnings.
Errors or warnings detected by the routine:
The maximum number of subdivisions allowed with the given workspace has been reached without the accuracy requirements being achieved. Look at the integrand in order to determine the integration difficulties. If necessary, another integrator, which is designed for handling the type of difficulty involved, must be used. Alternatively, consider relaxing the accuracy requirements specified by EPSABS
, or increasing the amount of workspace.
Round-off error prevents the requested tolerance from being achieved. Consider requesting less accuracy.
Extremely bad local behaviour of causes a very strong subdivision around one (or more) points of the interval. The same advice applies as in the case of .
D01AQF cannot guarantee, but in practice usually achieves, the following accuracy:
are user-specified absolute and relative error tolerances. Moreover, it returns the quantity ABSERR
which, in normal circumstances satisfies:
The time taken by D01AQF depends on the integrand and the accuracy required.
on exit, then you may wish to examine the contents of the array W
, which contains the end points of the sub-intervals used by D01AQF along with the integral contributions and error estimates over these sub-intervals.
denote the approximation to the value of the integral over the sub-interval [
] in the partition of
be the corresponding absolute error estimate. Then,
. The value of
is returned in
and the values
are stored consecutively in the
This example computes the Cauchy principal value of
9.1 Program Text
Program Text (d01aqfe.f90)
9.2 Program Data
9.3 Program Results
Program Results (d01aqfe.r)