# NAG Library Routine Document

## 1Purpose

s17aef returns the value of the Bessel function ${J}_{0}\left(x\right)$, via the function name.

## 2Specification

Fortran Interface
 Function s17aef ( x,
 Real (Kind=nag_wp) :: s17aef Integer, Intent (Inout) :: ifail Real (Kind=nag_wp), Intent (In) :: x
#include nagmk26.h
 double s17aef_ (const double *x, Integer *ifail)

## 3Description

s17aef evaluates an approximation to the Bessel function of the first kind ${J}_{0}\left(x\right)$.
Note:  ${J}_{0}\left(-x\right)={J}_{0}\left(x\right)$, so the approximation need only consider $x\ge 0$.
The routine is based on three Chebyshev expansions:
For $0,
 $J0x=∑′r=0arTrt, with ​t=2 x8 2 -1.$
For $x>8$,
 $J0x= 2πx P0xcosx-π4-Q0xsinx- π4 ,$
where ${P}_{0}\left(x\right)=\underset{r=0}{{\sum }^{\prime }}\phantom{\rule{0.25em}{0ex}}{b}_{r}{T}_{r}\left(t\right)$,
and ${Q}_{0}\left(x\right)=\frac{8}{x}\underset{r=0}{{\sum }^{\prime }}\phantom{\rule{0.25em}{0ex}}{c}_{r}{T}_{r}\left(t\right)$,
with $t=2{\left(\frac{8}{x}\right)}^{2}-1$.
For $x$ near zero, ${J}_{0}\left(x\right)\simeq 1$. This approximation is used when $x$ is sufficiently small for the result to be correct to machine precision.
For very large $x$, it becomes impossible to provide results with any reasonable accuracy (see Section 7), hence the routine fails. Such arguments contain insufficient information to determine the phase of oscillation of ${J}_{0}\left(x\right)$; only the amplitude, $\sqrt{\frac{2}{\pi \left|x\right|}}$, can be determined and this is returned on soft failure. The range for which this occurs is roughly related to machine precision; the routine will fail if  (see the Users' Note for your implementation for details).

## 4References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
Clenshaw C W (1962) Chebyshev Series for Mathematical Functions Mathematical tables HMSO

## 5Arguments

1:     $\mathbf{x}$ – Real (Kind=nag_wp)Input
On entry: the argument $x$ of the function.
2:     $\mathbf{ifail}$ – IntegerInput/Output
On entry: ifail must be set to $0$, $-1\text{​ or ​}1$. 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 $-1\text{​ or ​}1$ 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 $-\mathbf{1}\text{​ or ​}\mathbf{1}$ is used it is essential to test the value of ifail on exit.
On exit: ${\mathbf{ifail}}={\mathbf{0}}$ unless the routine detects an error or a warning has been flagged (see Section 6).

## 6Error Indicators and Warnings

If on entry ${\mathbf{ifail}}=0$ or $-1$, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
${\mathbf{ifail}}=1$
x is too large. On soft failure the routine returns the amplitude of the ${J}_{0}$ oscillation, $\sqrt{\frac{2}{\pi \left|x\right|}}$.
${\mathbf{ifail}}=-99$
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

## 7Accuracy

Let $\delta$ be the relative error in the argument and $E$ be the absolute error in the result. (Since ${J}_{0}\left(x\right)$ oscillates about zero, absolute error and not relative error is significant.)
If $\delta$ is somewhat larger than the machine precision (e.g., if $\delta$ is due to data errors etc.), then $E$ and $\delta$ are approximately related by:
 $E≃xJ1xδ$
(provided $E$ is also within machine bounds). Figure 1 displays the behaviour of the amplification factor $\left|x{J}_{1}\left(x\right)\right|$.
However, if $\delta$ is of the same order as machine precision, then rounding errors could make $E$ slightly larger than the above relation predicts.
For very large $x$, the above relation ceases to apply. In this region, ${J}_{0}\left(x\right)\simeq \sqrt{\frac{2}{\pi \left|x\right|}}\mathrm{cos}\left(x-\frac{\pi }{4}\right)$. The amplitude $\sqrt{\frac{2}{\pi \left|x\right|}}$ can be calculated with reasonable accuracy for all $x$, but $\mathrm{cos}\left(x-\frac{\pi }{4}\right)$ cannot. If $x-\frac{\pi }{4}$ is written as $2N\pi +\theta$ where $N$ is an integer and $0\le \theta <2\pi$, then $\mathrm{cos}\left(x-\frac{\pi }{4}\right)$ is determined by $\theta$ only. If $x\gtrsim {\delta }^{-1}$, $\theta$ cannot be determined with any accuracy at all. Thus if $x$ is greater than, or of the order of, the inverse of the machine precision, it is impossible to calculate the phase of ${J}_{0}\left(x\right)$ and the routine must fail.
Figure 1

## 8Parallelism and Performance

s17aef is not threaded in any implementation.

None.

## 10Example

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

### 10.1Program Text

Program Text (s17aefe.f90)

### 10.2Program Data

Program Data (s17aefe.d)

### 10.3Program Results

Program Results (s17aefe.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017