NAG CL Interface
s18efc (nag_bessel_k_nu)

1 Purpose

s18efc returns the value of the modified Bessel function K ν/4 x for real x>0 .

2 Specification

#include <nag.h>
double  s18efc (double x, Integer nu, NagError *fail)
The function may be called by the names: s18efc or nag_bessel_k_nu.

3 Description

s18efc evaluates an approximation to the modified Bessel function of the second kind K ν/4 x , where the order ν = -3 , -2 , -1 , 1 , 2 or 3 and x is real and positive. For negative orders the formula
K - ν / 4 x = K ν/4 x  
is used.

4 References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications

5 Arguments

1: x double Input
On entry: the argument x of the function.
Constraint: x>0.0 .
2: nu Integer Input
On entry: the argument ν of the function.
Constraint: 1 absnu 3 .
3: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

On entry, nu=value .
Constraint: 1 absnu 3 .
An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.
The evaluation has been abandoned due to the likelihood of overflow. The result is returned as zero.
On entry, x=value .
Constraint: x>0.0 .
The evaluation has been abandoned due to failure to satisfy the termination condition. The result is returned as zero.
The evaluation has been abandoned due to total loss of precision. The result is returned as zero.
The evaluation has been completed but some precision has been lost.

7 Accuracy

All constants in the underlying function are specified to approximately 18 digits of precision. If t denotes the number of digits of precision in the floating-point arithmetic being used, then clearly the maximum number of correct digits in the results obtained is limited by p = mint,18 . Because of errors in argument reduction when computing elementary functions inside the underlying function, the actual number of correct digits is limited, in general, by p-s , where s max1, log10x represents the number of digits lost due to the argument reduction. Thus the larger the value of x , the less the precision in the result.

8 Parallelism and Performance

s18efc is not threaded in any implementation.

9 Further Comments


10 Example

The example program reads values of the arguments x and ν from a file, evaluates the function and prints the results.

10.1 Program Text

Program Text (s18efce.c)

10.2 Program Data

Program Data (s18efce.d)

10.3 Program Results

Program Results (s18efce.r)