NAG Library Routine Document
s18cff returns a value of the scaled modified Bessel function via the routine name.
|Real (Kind=nag_wp)||:: ||s18cff|
|Integer, Intent (Inout)||:: ||
|Real (Kind=nag_wp), Intent (In)||:: ||
x|C Header Interface
const double *x,
s18cff evaluates an approximation to , where is a modified Bessel function of the first kind. The scaling factor removes most of the variation in .
The routine uses the same Chebyshev expansions as s18aff
, which returns the unscaled value of
Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
- 1: – Real (Kind=nag_wp)Input
On entry: the argument of the function.
- 2: – IntegerInput/Output
must be set to
. 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
is recommended. If the output of error messages is undesirable, then the value
is recommended. Otherwise, if you are not familiar with this argument, 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
Error Indicators and Warnings
There are no actual failure exits from this routine. ifail
is always set to zero. This argument is included for compatibility with other routines in this chapter.
Relative errors in the argument are attenuated when propagated into the function value. When the accuracy of the argument is essentially limited by the machine precision, the accuracy of the function value will be similarly limited by at most a small multiple of the machine precision.
Parallelism and Performance
s18cff is not threaded in any implementation.
This example reads values of the argument from a file, evaluates the function at each value of and prints the results.
Program Text (s18cffe.f90)
Program Data (s18cffe.d)
Program Results (s18cffe.r)