NAG Library Routine Document

s17gbf (struve_h1)


    1  Purpose
    7  Accuracy


s17gbf returns the value of the Struve function of order 1, H1x, via the function name.


Fortran Interface
Function s17gbf ( x, ifail)
Real (Kind=nag_wp):: s17gbf
Integer, Intent (Inout):: ifail
Real (Kind=nag_wp), Intent (In):: x
C Header Interface
#include nagmk26.h
double  s17gbf_ (const double *x, Integer *ifail)


s17gbf evaluates an approximation to the Struve function of order 1, H1x.
Please consult the NIST Digital Library of Mathematical Functions for a detailed discussion of the Struve function including special cases, transformations, relations and asymptotic approximations.
The approximation method used by this routine is based on Chebyshev expansions.


NIST Digital Library of Mathematical Functions
MacLeod A J (1996) MISCFUN, a software package to compute uncommon special functions ACM Trans. Math. Software (TOMS) 22(3) 288–301


1:     x – Real (Kind=nag_wp)Input
On entry: the argument x of the function.
Constraint: x 1 ε 2  where ε is the machine precision as returned by x02ajf.
2:     ifail – IntegerInput/Output
On entry: ifail must be set to 0, -1​ 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​ 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 -1​ or ​1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

Error Indicators and Warnings

If on entry 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:
x is too large and the routine returns zero.
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.
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.
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.


The Chebyshev coefficients used by this routine are internally represented to 20 digits of precision. Calling the number of digits of precision in the floating-point arithmetic being used t, then clearly the maximum number of correct digits in the results obtained is limited by p=mint,20.
Apart from this, rounding errors in internal arithmetic may result in a slight loss of accuracy, but it is reasonable to assume that the result is accurate to within a small multiple of the machine precision.

Parallelism and Performance

s17gbf is not threaded in any implementation.

Further Comments

For x > 1 ε 2 , H1x is asymptotically close to the Bessel function Y1x which is approximately zero to machine precision.


This example reads values of the argument x from a file, evaluates the function at each value of x and prints the results.

Program Text

Program Text (s17gbfe.f90)

Program Data

Program Data (s17gbfe.d)

Program Results

Program Results (s17gbfe.r)

GnuplotProduced by GNUPLOT 5.0 patchlevel 3 0 0.2 0.4 0.6 0.8 1 1.2 −10 −8 −6 −4 −2 0 2 4 6 8 10 H1(x) x Example Program Returns values for the Struve Function H1(x) "s17gbfe.r"
© The Numerical Algorithms Group Ltd, Oxford, UK. 2017