# NAG Library Routine Document

## 1Purpose

s21baf returns a value of an elementary integral, which occurs as a degenerate case of an elliptic integral of the first kind, via the function name.

## 2Specification

Fortran Interface
 Function s21baf ( x, y,
 Real (Kind=nag_wp) :: s21baf Integer, Intent (Inout) :: ifail Real (Kind=nag_wp), Intent (In) :: x, y
#include nagmk26.h
 double s21baf_ (const double *x, const double *y, Integer *ifail)

## 3Description

s21baf calculates an approximate value for the integral
 $RC x,y = 12 ∫ 0 ∞ dt t+y . t+x$
where $x\ge 0$ and $y\ne 0$.
This function, which is related to the logarithm or inverse hyperbolic functions for $y and to inverse circular functions if $x, arises as a degenerate form of the elliptic integral of the first kind. If $y<0$, the result computed is the Cauchy principal value of the integral.
The basic algorithm, which is due to Carlson (1979) and Carlson (1988), is to reduce the arguments recursively towards their mean by the system:
 $x0=x y0=y μn=xn+2yn/3, Sn=yn-xn/3μn λn=yn+2xnyn xn+1=xn+λn/4, yn+1=yn+λn/4.$
The quantity $\left|{S}_{n}\right|$ for $n=0,1,2,3,\dots \text{}$ decreases with increasing $n$, eventually $\left|{S}_{n}\right|\sim 1/{4}^{n}$. For small enough ${S}_{n}$ the required function value can be approximated by the first few terms of the Taylor series about the mean. That is
 $RCx,y=1+3Sn210+Sn37+3Sn48+9Sn522 /μn.$
The truncation error involved in using this approximation is bounded by $16{\left|{S}_{n}\right|}^{6}/\left(1-2\left|{S}_{n}\right|\right)$ and the recursive process is stopped when ${S}_{n}$ is small enough for this truncation error to be negligible compared to the machine precision.
Within the domain of definition, the function value is itself representable for all representable values of its arguments. However, for values of the arguments near the extremes the above algorithm must be modified so as to avoid causing underflows or overflows in intermediate steps. In extreme regions arguments are prescaled away from the extremes and compensating scaling of the result is done before returning to the calling program.

## 4References

Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
Carlson B C (1979) Computing elliptic integrals by duplication Numerische Mathematik 33 1–16
Carlson B C (1988) A table of elliptic integrals of the third kind Math. Comput. 51 267–280

## 5Arguments

1:     $\mathbf{x}$ – Real (Kind=nag_wp)Input
2:     $\mathbf{y}$ – Real (Kind=nag_wp)Input
On entry: the arguments $x$ and $y$ of the function, respectively.
Constraint: ${\mathbf{x}}\ge 0.0$ and ${\mathbf{y}}\ne 0.0$.
3:     $\mathbf{ifail}$ – 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:
${\mathbf{ifail}}=1$
 On entry, ${\mathbf{x}}<0.0$; the function is undefined.
${\mathbf{ifail}}=2$
 On entry, ${\mathbf{y}}=0.0$; the function is undefined.
On soft failure the routine returns zero.
${\mathbf{ifail}}=-99$
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

In principle the routine is capable of producing full machine precision. However round-off errors in internal arithmetic will result in slight loss of accuracy. This loss should never be excessive as the algorithm does not involve any significant amplification of round-off error. It is reasonable to assume that the result is accurate to within a small multiple of the machine precision.

## 8Parallelism and Performance

s21baf is not threaded in any implementation.

You should consult the S Chapter Introduction which shows the relationship of this function to the classical definitions of the elliptic integrals.

## 10Example

This example simply generates a small set of nonextreme arguments which are used with the routine to produce the table of low accuracy results.

### 10.1Program Text

Program Text (s21bafe.f90)

None.

### 10.3Program Results

Program Results (s21bafe.r)

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