# NAG CL Interfaces21bbc (ellipint_​symm_​1)

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## 1Purpose

s21bbc returns a value of the symmetrised elliptic integral of the first kind.

## 2Specification

 #include
 double s21bbc (double x, double y, double z, NagError *fail)
The function may be called by the names: s21bbc, nag_specfun_ellipint_symm_1 or nag_elliptic_integral_rf.

## 3Description

s21bbc calculates an approximation to the integral
 $RF(x,y,z)=12∫0∞dt (t+x)(t+y)(t+z)$
where $x$, $y$, $z\ge 0$ and at most one is zero.
The basic algorithm, which is due to Carlson (1979) and Carlson (1988), is to reduce the arguments recursively towards their mean by the rule:
• ${x}_{0}=\mathrm{min}\phantom{\rule{0.125em}{0ex}}\left(x,y,z\right)$, $\text{ }{z}_{0}=\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(x,y,z\right)$,
• ${y}_{0}=\text{}$ remaining third intermediate value argument.
(This ordering, which is possible because of the symmetry of the function, is done for technical reasons related to the avoidance of overflow and underflow.)
 $μn = (xn+yn+zn)/3 Xn = (1-xn)/μn Yn = (1-yn)/μn Zn = (1-zn)/μn λn = xnyn+ynzn+znxn xn+1 = (xn+λn)/4 yn+1 = (yn+λn)/4 zn+1 = (zn+λn)/4$
${\epsilon }_{n}=\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(|{X}_{n}|,|{Y}_{n}|,|{Z}_{n}|\right)$ and the function may be approximated adequately by a fifth order power series:
 $RF(x,y,z)=1μn (1-E210+E2224-3E2E344+E314)$
where ${E}_{2}={X}_{n}{Y}_{n}+{Y}_{n}{Z}_{n}+{Z}_{n}{X}_{n}$, ${E}_{3}={X}_{n}{Y}_{n}{Z}_{n}$.
The truncation error involved in using this approximation is bounded by ${\epsilon }_{n}^{6}/4\left(1-{\epsilon }_{n}\right)$ and the recursive process is stopped when this truncation error is negligible compared with 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

NIST Digital Library of Mathematical Functions
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: $\mathbf{x}$double Input
2: $\mathbf{y}$double Input
3: $\mathbf{z}$double Input
On entry: the arguments $x$, $y$ and $z$ of the function.
Constraint: ${\mathbf{x}}$, y, ${\mathbf{z}}\ge 0.0$ and only one of x, y and z may be zero.
4: $\mathbf{fail}$NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

## 6Error Indicators and Warnings

NE_ALLOC_FAIL
Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
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.
See Section 7.5 in the Introduction to the NAG Library CL Interface for further information.
NE_NO_LICENCE
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library CL Interface for further information.
NE_REAL_ARG_EQ
On entry, ${\mathbf{x}}=⟨\mathit{\text{value}}⟩$, ${\mathbf{y}}=⟨\mathit{\text{value}}⟩$ and ${\mathbf{z}}=⟨\mathit{\text{value}}⟩$.
Constraint: at most one of x, y and z is $0.0$.
The function is undefined and returns zero.
NE_REAL_ARG_LT
On entry, ${\mathbf{x}}=⟨\mathit{\text{value}}⟩$, ${\mathbf{y}}=⟨\mathit{\text{value}}⟩$ and ${\mathbf{z}}=⟨\mathit{\text{value}}⟩$.
Constraint: ${\mathbf{x}}\ge 0.0$ and ${\mathbf{y}}\ge 0.0$ and ${\mathbf{z}}\ge 0.0$.
The function is undefined.

## 7Accuracy

In principle s21bbc 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

s21bbc 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.
If two arguments are equal, the function reduces to the elementary integral ${R}_{C}$, computed by s21bac.

## 10Example

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

### 10.1Program Text

Program Text (s21bbce.c)

None.

### 10.3Program Results

Program Results (s21bbce.r)