NAG CL Interface
s18cfc (bessel_​i1_​scaled)

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1 Purpose

s18cfc returns a value of the scaled modified Bessel function e-|x|I1(x).

2 Specification

#include <nag.h>
double  s18cfc (double x)
The function may be called by the names: s18cfc, nag_specfun_bessel_i1_scaled or nag_bessel_i1_scaled.

3 Description

s18cfc evaluates an approximation to e-|x|I1(x), where I1 is a modified Bessel function of the first kind. The scaling factor e-|x| removes most of the variation in I1(x).
The function uses the same Chebyshev expansions as s18afc, which returns the unscaled value of I1(x).

4 References

NIST Digital Library of Mathematical Functions

5 Arguments

1: x double Input
On entry: the argument x of the function.

6 Error Indicators and Warnings


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

s18cfc 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 (s18cfce.c)

10.2 Program Data

Program Data (s18cfce.d)

10.3 Program Results

Program Results (s18cfce.r)