NAG Library Routine Document

s15aff (dawson)

1
Purpose

s15aff returns a value for Dawson's Integral, Fx, via the function name.

2
Specification

Fortran Interface
Function s15aff ( x, ifail)
Real (Kind=nag_wp):: s15aff
Integer, Intent (Inout):: ifail
Real (Kind=nag_wp), Intent (In):: x
C Header Interface
#include <nagmk26.h>
double  s15aff_ (const double *x, Integer *ifail)

3
Description

s15aff evaluates an approximation for Dawson's Integral
Fx = e-x2 0x et2 dt .  
The routine is based on two Chebyshev expansions:
For 0<x4,
Fx = x r=0 ar Tr t ,   where   t=2 x4 2 -1 .  
For x>4,
Fx = 1x r=0 br Tr t ,   where   t=2 4x 2 -1 .  
For x near zero, Fxx, and for x large, Fx12x. These approximations are used for those values of x for which the result is correct to machine precision.

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 – IntegerInput/Output
On entry: ifail must be set to 0, -1 or 1. 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 -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 failure exits from this routine.

7
Accuracy

Let δ and ε be the relative errors in the argument and result respectively.
If δ is considerably greater than the machine precision (i.e., if δ is due to data errors etc.), then ε and δ are approximately related by:
ε x 1-2xFx Fx δ.  
The following graph shows the behaviour of the error amplification factor x 1-2xFx Fx :
Figure 1
Figure 1
However, if δ is of the same order as machine precision, then rounding errors could make ε somewhat larger than the above relation indicates. In fact ε will be largely independent of x or δ, but will be of the order of a few times the machine precision.

8
Parallelism and Performance

s15aff is not threaded in any implementation.

9
Further Comments

None.

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 (s15affe.f90)

10.2
Program Data

Program Data (s15affe.d)

10.3
Program Results

Program Results (s15affe.r)