NAG FL Interface
s18ctf (bessel_​i1_​scaled_​vector)

1 Purpose

s18ctf returns an array of values of the scaled modified Bessel function e-xI1x.

2 Specification

Fortran Interface
Subroutine s18ctf ( n, x, f, ifail)
Integer, Intent (In) :: n
Integer, Intent (Inout) :: ifail
Real (Kind=nag_wp), Intent (In) :: x(n)
Real (Kind=nag_wp), Intent (Out) :: f(n)
C Header Interface
#include <nag.h>
void  s18ctf_ (const Integer *n, const double x[], double f[], Integer *ifail)
The routine may be called by the names s18ctf or nagf_specfun_bessel_i1_scaled_vector.

3 Description

s18ctf evaluates an approximation to e-xiI1xi, where I1 is a modified Bessel function of the first kind for an array of arguments xi, for i=1,2,,n. The scaling factor e-x removes most of the variation in I1x.
The routine uses the same Chebyshev expansions as s18atf, which returns an array of the unscaled values of I1x.

4 References

NIST Digital Library of Mathematical Functions

5 Arguments

1: n Integer Input
On entry: n, the number of points.
Constraint: n0.
2: xn Real (Kind=nag_wp) array Input
On entry: the argument xi of the function, for i=1,2,,n.
3: fn Real (Kind=nag_wp) array Output
On exit: e-xiI1xi, the function values.
4: 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

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, n=value.
Constraint: n0.
ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
ifail=-399
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
ifail=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

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

s18ctf is not threaded in any implementation.

9 Further Comments

None.

10 Example

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

10.1 Program Text

Program Text (s18ctfe.f90)

10.2 Program Data

Program Data (s18ctfe.d)

10.3 Program Results

Program Results (s18ctfe.r)