NAG FL Interface
s18cff (bessel_​i1_​scaled)

1 Purpose

s18cff returns a value of the scaled modified Bessel function e-xI1x via the routine name.

2 Specification

Fortran Interface
Function s18cff ( x, ifail)
Real (Kind=nag_wp) :: s18cff
Integer, Intent (Inout) :: ifail
Real (Kind=nag_wp), Intent (In) :: x
C Header Interface
#include <nag.h>
double  s18cff_ (const double *x, Integer *ifail)
The routine may be called by the names s18cff or nagf_specfun_bessel_i1_scaled.

3 Description

s18cff evaluates an approximation to e-xI1x, where I1 is a modified Bessel function of the first kind. The scaling factor e-x removes most of the variation in I1x.
The routine uses the same Chebyshev expansions as s18aff, which returns the unscaled value of I1x.

4 References

NIST Digital Library of Mathematical Functions

5 Arguments

1: x Real (Kind=nag_wp) Input
On entry: the argument x of the function.
2: ifail Integer Input/Output
On entry: ifail must be set to 0, -1 or 1. If you are unfamiliar with this argument you should refer to Section 4 in the Introduction to the NAG Library FL Interface 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

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.

7 Accuracy

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.

8 Parallelism and Performance

s18cff is not threaded in any implementation.

9 Further Comments


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 (s18cffe.f90)

10.2 Program Data

Program Data (s18cffe.d)

10.3 Program Results

Program Results (s18cffe.r)