NAG Library Routine Document

s17aff (bessel_j1_real)

1
Purpose

s17aff returns the value of the Bessel function J1x, via the function name.

2
Specification

Fortran Interface
Function s17aff ( x, ifail)
Real (Kind=nag_wp):: s17aff
Integer, Intent (Inout):: ifail
Real (Kind=nag_wp), Intent (In):: x
C Header Interface
#include <nagmk26.h>
double  s17aff_ (const double *x, Integer *ifail)

3
Description

s17aff evaluates an approximation to the Bessel function of the first kind J1x.
Note:  J1-x=-J1x, so the approximation need only consider x0.
The routine is based on three Chebyshev expansions:
For 0<x8,
J1x=x8r=0arTrt,   with ​t=2 x8 2-1.  
For x>8,
J1x=2πx P1xcosx-3π4-Q1xsinx-3π4  
where P1x=r=0brTrt,
and Q1x= 8xr=0crTrt,
with t=2 8x 2-1.
For x near zero, J1x x2 . This approximation is used when x is sufficiently small for the result to be correct to machine precision.
For very large x, it becomes impossible to provide results with any reasonable accuracy (see Section 7), hence the routine fails. Such arguments contain insufficient information to determine the phase of oscillation of J1x; only the amplitude, 2πx , can be determined and this is returned on soft failure. The range for which this occurs is roughly related to machine precision; the routine will fail if x1/machine precision (see the Users' Note for your implementation for details).

4
References

NIST Digital Library of Mathematical Functions
Clenshaw C W (1962) Chebyshev Series for Mathematical Functions Mathematical tables HMSO

5
Arguments

1:     x – Real (Kind=nag_wp)Input
On entry: the argument x of the function.
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).

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:
ifail=1
On entry, x=value.
Constraint: xvalue.
x is too large, the function returns the amplitude of the J1 oscillation, 2/πx.
ifail=-99
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.
ifail=-399
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.
ifail=-999
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

7
Accuracy

Let δ be the relative error in the argument and E be the absolute error in the result. (Since J1x oscillates about zero, absolute error and not relative error is significant.)
If δ is somewhat larger than machine precision (e.g., if δ is due to data errors etc.), then E and δ are approximately related by:
ExJ0x-J1xδ  
(provided E is also within machine bounds). Figure 1 displays the behaviour of the amplification factor xJ0x-J1x.
However, if δ is of the same order as machine precision, then rounding errors could make E slightly larger than the above relation predicts.
For very large x, the above relation ceases to apply. In this region, J1x 2πx cosx- 3π4. The amplitude 2πx  can be calculated with reasonable accuracy for all x, but cosx- 3π4 cannot. If x- 3π4  is written as 2Nπ+θ where N is an integer and 0θ<2π, then cosx- 3π4 is determined by θ only. If xδ-1, θ cannot be determined with any accuracy at all. Thus if x is greater than, or of the order of, the reciprocal of machine precision, it is impossible to calculate the phase of J1x and the routine must fail.
Figure 1
Figure 1

8
Parallelism and Performance

s17aff is not threaded in any implementation.

9
Further Comments

None.

10
Example

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

10.1
Program Text

Program Text (s17affe.f90)

10.2
Program Data

Program Data (s17affe.d)

10.3
Program Results

Program Results (s17affe.r)

GnuplotProduced by GNUPLOT 4.6 patchlevel 3 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 −20 −10 0 10 20 J1(x) x Example Program Returned Values for the Bessel Function J1(x) gnuplot_plot_1