QDAG1D
Integrates a function with a possible internal or endpoint singularity.
Required Arguments
F — User-supplied FUNCTION to be integrated. The form is F(X [, …]), where
Function Return Value
F — The function value. (Output)
Required Arguments
X — Independent variable. (Input)
Optional Arguments
FCN_DATA — A derived type, s_fcn_data, which may be used to pass additional information to/from the user-supplied function. For a detailed description of this argument see FCN_DATA below.
F must be declared EXTERNAL in the calling program.
A — Lower limit of integration. (Input)
B — Upper limit of integration. The relative values of A and B are interpreted properly. Thus if one exchanges A and B, the sign of the answer is changed. When the integrand is positive, the sign of the result is the same as the sign of B – A. (Input)
RESULT — Estimate of the integral from A to B of F. (Output)
Optional Arguments
ERRABS — Absolute error tolerance. See Comment 1 for a discussion on the error tolerances. (Input)
Default: ERRABS = 0.0.
ERRFRAC — A fraction expressing the (number of correct digits of accuracy desired)/(number of digits of achievable precision). See Comment 1 for a discussion on the error tolerances. (Input)
Default: ERRFRAC = 0.75.
ERRREL —The error tolerance relative to the value of the integral. See Comment 1 for a discussion on the error tolerances. (Input)
Default: ERRREL = 0.0.
ERRPOST — An a posteriori estimate of the absolute value of the error committed while evaluating the integrand. This value may be computed during the evaluation of the integrand. When this optional argument is used, FCN_DATA must also be used as FCN_DATA%RDATA(1) will be used to pass the newly calculated value of ERRPOST back from the evaluator, F. In this case, the user should not use FCN_DATA%RDATA(1) for passing other data. (Input)
Default: ERRPOST = 0.0.
ERRPRIOR— An a priori estimate of the absolute value of the relative error expected to be committed while evaluating the integrand. Changes to this value are not detected during evaluation of the integral. (Input)
Default: ERRPRIOR = 1.19e-7 for single precision and 2.22d-16 for double precision.
MAXFCN —The maximum number of function values to use to compute the integral. (Input)
Default: The number of function values is not bounded.
SINGULARITY —The real part of the abscissa of a singularity or discontinuity in the integrand. If this option is used, SINGULARITY_TYPE must also be used. (Input)
Default: It is assumed that there is no singularity in the integrand so SINGULARITY is not set. It is an error to set SINGULARITY without also setting SINGULARITY_TYPE.
SINGULARITY_TYPE—A signed integer specifying the type of singularity which occurs in the integrand. If the singularity has a leading term of the form xα where α is not an integer, if α is “large” or has the form α = (2n-1)/2 where n is a nonnegative integer, or the singularity is well outside the interval, set SINGULARITY_TYPE to a positive integer. Otherwise, set SINGULARITY_TYPE to a negative integer. (Input)
Default: It is assumed that there is no singularity in the integrand so SINGULARITY_TYPE is not set. It is an error to set SINGULARITY_TYPE without also setting SINGULARITY.
FCN_DATA — A derived type, s_fcn_data, which may be used to pass additional information to/from the user-supplied function. The derived type, s _fcn_data, is defined as:
type s_fcn_data
real(kind(1e0)), pointer, dimension(:) :: rdata
integer, pointer, dimension(:) :: idata
end type
in module mp_types. The double precision counterpart to s_fcn_data is named d_fcn_data. The user must include a use mp_types statement in the calling program to define this derived type. Note that if this optional argument is used then this argument must also be used in the user-supplied function. (Input/Output)
NEVAL — Number of function evaluations used to calculate the integral. (Output)
ERREST — An estimate of the upper bound of the magnitude of the difference between RESULT and the true value of the integral. (Output)
ISTATUS — A status flag indicating the error criteria which was satisfied on exit.
ISTATUS = -1 indicates normal termination with either the absolute or relative error tolerance criteria satisfied.
ISTATUS = -2 indicates normal termination with neither the absolute nor the relative error tolerance criteria satisfied, but the error tolerance based on the locally achievable precision is satisfied.
ISTATUS = -3 indicates normal termination with none of the error tolerance criteria satisfied.
ISTATUS = any value other than the above indicates abnormal termination due to an error condition. (Output)
FORTRAN 90 Interface
Generic: CALL QDAG1D (F,A, B, RESULT [, …])
Specific: The specific interface names are S_QDAG1D and D_QDAG1D.
Description
QDAG1D is based on the JPL Library routine SINT1. The integral is estimated using quadrature formulae due to T. N. L. Patterson (1968). Patterson described a family of formulae in which the kth formula used all the integrand values used in the k-1st formula, and added 2k-1 new integrand values in an optimal way. The first formula is the midpoint rule, the second is the three point Gauss formula, and the third is the seven point Kronrod formula. Formulae of this family of higher degree had not previously been described. This program uses formulae up to k = 8.
An error estimate is obtained by comparing the values of the integral estimated by two adjacent formulae, examining differences up to the fifteenth order, integrating round-off error, integrating error declared to have been committed during computation of the integrand, integrating a first order estimate of the effect round-off error in the abscissa has on integrand values, and including errors in the limits. The latter four methods are also used to derive a bound on the achievable precision.
If the integral over an interval cannot be estimated with sufficient accuracy, the interval is subdivided. The difference table is used to discover whether the integral is difficult to compute because the integrand is too complex or has singular behavior. In the former case, the estimated error, requested error tolerance, and difference table are used to choose a step size.
In the latter case, the difference table is used in a search algorithm to find the abscissa of the singular behavior. If the singular behavior is discovered on the end of an interval, a change of independent variable is applied to reduce the strength of the singularity.
The program also uses the difference table to detect nonintegrable singularities, jump discontinuities, and computational noise.
Comments
1. The user provides the absolute error tolerance through optional argument ERRABS. Optional argument ERRFRAC represents the ratio of the (number of correct digits of accuracy desired) to (number of digits of achievable precision). Optional argument ERRREL represents the error tolerance relative to the value of the integral. The internal value for ERRFRAC is bounded between .5 and 1. By default, ERRABS and ERRREL are set to 0.0 and ERRFRAC is set to .75. These default values usually provide all the accuracy that can be obtained efficiently.
The error tolerance relative to the value of the integral is applied globally (over the entire region of integration) rather than locally (one step at a time). This policy provides true control of error relative to the value of the integral when the integrand is not sign definite, as well as when the integrand is sign definite. To apply the criterion of error tolerance relative to the value of the integral, the value of the integral over the entire region, estimated without refinement of the region, is used to derive an absolute error tolerance that may be applied locally. If the preliminary estimate of the value of the integral is significantly in error, and the least restrictive error tolerance is relative to the value of the integral, the cost of computing the integral will be larger than the cost of computing the integral to the same degree of accuracy using appropriate values of either of the other tolerance criteria. The preliminary estimate of the integral may be significantly in error if the integrand is not sign definite or has large variation.
2. Optional arguments SINGULARITY and SINGULARITY_TYPE provide the user with a means to give the routine information about the location and type of any known singularity of the integrand. When an integrand appears to have singular behavior at the end of the interval, a transformation of the variable of integration is applied to reduce the strength of the singularity. When an integrand appears to have singular behavior inside the interval, the abscissa of the singularity is determined as precisely as necessary, depending on the error tolerance, and the interval is subdivided. The discovery of singular behavior and determination of the abscissa of singular behavior are expensive. If the user knows of the existence of a singularity, the efficiency of computation of the integral may be improved by requesting an immediate transformation of the independent variable or subdivision of the interval. It is recommended that the user select these optional arguments for all singularities, even those outside [A, B]. If the singularity has a leading term of the form xα where α is not an integer, if α is “large” or has the form α = (2n-1)/2 where n is a nonnegative integer, or the singularity is well outside the interval, set SINGULARITY_TYPE to a positive value. Otherwise, set SINGULARITY_TYPE to a negative value. The meaning of “large” depends on the rest of the integrand and the length of the interval. For the typical case, a value of about 2 is considered “large”. For a singularity of the form xαlog x use the above rule, even if α is an integer. For other types of singularities make a reasonable guess based on the above. If several similar integrals are to be computed, some experimentation may be useful.
When SINGULARITY_TYPE is positive, a transformation of the form T = TA + (X ‑ TA)2 /(TB ‑ TA) is applied, where TA is the abscissa of the singularity and TB is the end of the interval. If TA is outside the interval, TB will be the end of the interval farthest from TA. If TA is inside the interval, the interval will immediately be subdivided at TA, and both parts will be separately integrated with TB equal to each end of the original interval, respectively. When SINGULARITY_TYPE is negative, a transformation of the form T = TA + (X ‑ TA)4 / (TB ‑ TA)3 is applied, with TA and TB as above.
If the integrand has singularities at more than one abscissa within the region, or more than one pole near the real axis such that the real parts are within the region of integration, then the interval should be subdivided at the abscissa of the singularities or the real parts of the poles, and the integrals should be computed as separate problems, with the results summed.
Examples
The value of
is estimated. Note that the optional arguments SINGULARITY and SINGULARITY_TYPE are used.
USE QDAG1D_INT
USE UMACH_INT
IMPLICIT NONE
! Declare variables
INTEGER NOUT, SINGULARITY_TYPE
REAL A, B, ERREST, F, RESULT, SINGULARITY
EXTERNAL F
! Get output unit number
CALL UMACH (2, NOUT)
! Set limits of integration
A = 0.0
B = 1.0
! Set singularity value and type
SINGULARITY = 0.0
SINGULARITY_TYPE = -1
CALL QDAG1D ( F, A, B, RESULT, SINGULARITY=SINGULARITY, &
SINGULARITY_TYPE=SINGULARITY_TYPE, ERREST=ERREST)
! Print the results
WRITE(NOUT,*)'Result = ', RESULT
WRITE(NOUT,9999) ERREST
9999 FORMAT('Error Estimate = ', 1PE9.1)
END
REAL FUNCTION F (X)
REAL X
REAL ALOG, SQRT
INTRINSIC ALOG, SQRT
F = ALOG(X)/SQRT(X)
RETURN
END
Result = -4.0
Error Estimate = 6.0E-07
The value of
is estimated. Note that the optional argument FCN_DATA is used to set the value of k = 2 in the user-supplied function, F.
USE QDAG1D_INT
USE UMACH_INT
USE MP_TYPES
IMPLICIT NONE
! Declare variables
INTEGER NOUT
REAL A, B, ERREST, F, RESULT
REAL, TARGET :: RDATA(1)
TYPE (S_FCN_DATA) USER_DATA
EXTERNAL F
! Get output unit number
CALL UMACH (2, NOUT)
! Set limits of integration
A = 1.0
B = 2.0
! Set IPARAM
RDATA(1) = 2.0
USER_DATA%RDATA=>RDATA
CALL QDAG1D ( F, A, B, RESULT, FCN_DATA=USER_DATA, ERREST=ERREST)
! Print the results
WRITE(NOUT,*)'Result = ', RESULT
WRITE(NOUT,9999) ERREST
9999 FORMAT('Error Estimate = ', 1PE9.1)
END
REAL FUNCTION F (X, FCN_DATA)
USE MP_TYPES
TYPE (S_FCN_DATA) FCN_DATA
REAL X
F = 2.0 * X + FCN_DATA%RDATA(1) * X
RETURN
END
Result = 6.0
Error Estimate = 1.2E-06
