public static s17af( x, out ifail )
|Visual Basic (Declaration)|
Public Shared Function s17af ( _ x As , _ < > ByRef ifail As _ ) As
public: static s17af( x, [ ] % ifail )
static member s17af : x: * ifail: byref ->
On entry: the argument of the function.
On exit: unless the method detects an error (see [Error Indicators and Warnings]).
s17af evaluates an approximation to the Bessel function of the first kind .
Note: , so the approximation need only consider .
The method is based on three Chebyshev expansions:
For near zero, . This approximation is used when is sufficiently small for the result to be correct to machine precision.
For very large , it becomes impossible to provide results with any reasonable accuracy (see [Accuracy]), hence the method fails. Such arguments contain insufficient information to determine the phase of oscillation of ; only the amplitude, , can be determined and this is returned on soft failure. The range for which this occurs is roughly related to machine precision; the method will fail if (see the Library Constants section of the Introduction for details).
Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
Errors or warnings detected by the method:
x is too large. On failure the method returns the amplitude of the oscillation, .
Let be the relative error in the argument and be the absolute error in the result. (Since oscillates about zero, absolute error and not relative error is significant.)
If is somewhat larger than machine precision (e.g., if is due to data errors etc.), then and are approximately related by:
(provided is also within machine bounds). Figure 1 displays the behaviour of the amplification factor .
However, if is of the same order as machine precision, then rounding errors could make slightly larger than the above relation predicts.
For very large , the above relation ceases to apply. In this region, . The amplitude can be calculated with reasonable accuracy for all , but cannot. If is written as where is an integer and , then is determined by only. If , cannot be determined with any accuracy at all. Thus if is greater than, or of the order of, the reciprocal of machine precision, it is impossible to calculate the phase of and the method must fail.
This example reads values of the argument from a file, evaluates the function at each value of and prints the results.