nag_airy_bi_deriv (s17akc) (PDF version)
s Chapter Contents
s Chapter Introduction
NAG Library Manual

NAG Library Function Document

nag_airy_bi_deriv (s17akc)

+ Contents

    1  Purpose
    7  Accuracy

1  Purpose

nag_airy_bi_deriv (s17akc) returns a value for the derivative of the Airy function Bix.

2  Specification

#include <nag.h>
#include <nags.h>
double  nag_airy_bi_deriv (double x, NagError *fail)

3  Description

nag_airy_bi_deriv (s17akc) calculates an approximate value for the derivative of the Airy function Bix. It is based on a number of Chebyshev expansions.
For x<-5,
Bix=-x4 -atsinz+btζcosz ,
where z= π4+ζ, ζ= 23-x3 and at and bt are expansions in the variable t=-2 5x 3-1.
For -5x0,
Bix=3x2ft+gt,
where f and g are expansions in t=-2 x5 3-1.
For 0<x<4.5,
Bix=e3x/2yt,
where yt is an expansion in t=4x/9-1.
For 4.5x<9,
Bix=e21x/8ut,
where ut is an expansion in t=4x/9-3.
For x9,
Bix=x4ezvt,
where z= 23x3 and vt is an expansion in t=2 18z-1.
For x< the square of the machine precision, the result is set directly to Bi0. This saves time and avoids possible underflows in calculation.
For large negative arguments, it becomes impossible to calculate a result for the oscillating function with any accuracy so the function must fail. This occurs for x<- πε 4/7 , where ε is the machine precision.
For large positive arguments, where Bi grows in an essentially exponential manner, there is a danger of overflow so the function must fail.

4  References

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

5  Arguments

1:     xdoubleInput
On entry: the argument x of the function.
2:     failNagError *Input/Output
The NAG error argument (see Section 3.6 in the Essential Introduction).

6  Error Indicators and Warnings

NE_INTERNAL_ERROR
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.
NE_REAL_ARG_GT
On entry, x=value.
Constraint: xvalue.
x is too large and positive. The function returns zero.
NE_REAL_ARG_LT
On entry, x=value.
Constraint: xvalue.
x is too large and negative. The function returns zero.

7  Accuracy

For negative arguments the function is oscillatory and hence absolute error is appropriate. In the positive region the function has essentially exponential behaviour and hence relative error is needed. The absolute error, E, and the relative error ε, are related in principle to the relative error in the argument δ, by
E x2 Bix δ ε x2 Bix Bix δ.
In practice, approximate equality is the best that can be expected. When δ, ε or E is of the order of the machine precision, the errors in the result will be somewhat larger.
For small x, positive or negative, errors are strongly attenuated by the function and hence will effectively be bounded by the machine precision.
For moderate to large negative x, the error is, like the function, oscillatory. However, the amplitude of the absolute error grows like x7/4π . Therefore it becomes impossible to calculate the function with any accuracy if x7/4> πδ .
For large positive x, the relative error amplification is considerable: εδx3. However, very large arguments are not possible due to the danger of overflow. Thus in practice the actual amplification that occurs is limited.

8  Parallelism and Performance

Not applicable.

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 (s17akce.c)

10.2  Program Data

Program Data (s17akce.d)

10.3  Program Results

Program Results (s17akce.r)

Produced by GNUPLOT 4.4 patchlevel 0 -2 0 2 4 6 8 10 -10 -8 -6 -4 -2 0 2 4 Bi(x) x Example Program Returns a Value for the Derivative of the Airy Function Bi(x)

nag_airy_bi_deriv (s17akc) (PDF version)
s Chapter Contents
s Chapter Introduction
NAG Library Manual

© The Numerical Algorithms Group Ltd, Oxford, UK. 2014