NAG Library Function Document

nag_bessel_j1 (s17afc)

+− Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Further Comments

+− 9 Example

9.1 Program Text

9.2 Program Data

9.3 Program Results

1 Purpose

nag_bessel_j1 (s17afc) returns the value of the Bessel function

J_{1} (x)

2 Specification

#include <nag.h>

#include <nags.h>

double

nag_bessel_j1 (double x, NagError *fail)

3 Description

nag_bessel_j1 (s17afc) evaluates an approximation to the Bessel function of the first kind

J_{1} (x)

The function is based on Chebyshev expansions.

For

x

near zero,

J_{1} (x) ≃ x / 2

. 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 8), hence the function fails. Such arguments contain insufficient information to determine the phase of oscillation of

J_{1} (x)

; only the amplitude,

\sqrt{2 / π |x|}

, can be determined. The range for which this occurs is roughly related to the machine precision.

4 References

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

5 Arguments

1: x – doubleInput: On entry: the argument $x$ of the function.
2: fail – NagError *Input/Output: The NAG error argument (see Section 3.6 in the Essential Introduction).

6 Error Indicators and Warnings

NE_REAL_ARG_GT: On entry, $x = 〈value〉$ .
Constraint: $x \leq 〈value〉$ . x is too large. The function returns the amplitude of the $j_{1}$ oscillation, $\sqrt{2 / π |x|}$ .

7 Accuracy

Let

δ

be the relative error in the argument and

E

be the absolute error in the result. (Since

J_{1} (x)

oscillates about zero, absolute error and not relative error is significant.)

δ

is somewhat larger than machine precision (e.g., if

δ

is due to data errors etc.), then

E

and

δ

are approximately related by

E ≃ |{x J}_{0} (x) - J_{1} (x)| δ

(provided

E

is also within machine bounds).

However, if

δ

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_{1} (x) ≃ \sqrt{2 / π |x|} \cos (x - 3 π / 4)

. The amplitude

\sqrt{2 / π |x|}

can be calculated with reasonable accuracy for all

x

, but

\cos (x - 3 π / 4)

cannot. If

x - 3 π / 4

is written as

2 N π + θ

where

N

is an integer and

0 \leq θ < 2 π

, then

\cos (x - 3 π / 4)

is determined by

θ

only. If

x ≳ δ^{- 1}

θ

cannot be determined with any accuracy at all. Thus if

x

is greater than, or of the order of, machine precision, it is impossible to calculate the phase of

J_{1} (x)

and the function must fail.

8 Further Comments

None.

9 Example

The following program reads values of the argument

x

from a file, evaluates the function at each value of

x

and prints the results.

NAG Library Function Documentnag_bessel_j1 (s17afc)