NAG CL Interface
s21bec (ellipint_​legendre_​1)

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1 Purpose

s21bec returns a value of the classical (Legendre) form of the incomplete elliptic integral of the first kind.

2 Specification

#include <nag.h>
double  s21bec (double phi, double dm, NagError *fail)
The function may be called by the names: s21bec, nag_specfun_ellipint_legendre_1 or nag_elliptic_integral_f.

3 Description

s21bec calculates an approximation to the integral
F(ϕm) = 0ϕ (1-msin2θ) -12 dθ ,  
where 0ϕ π2 , msin2ϕ1 and m and sinϕ may not both equal one.
The integral is computed using the symmetrised elliptic integrals of Carlson (Carlson (1979) and Carlson (1988)). The relevant identity is
F(ϕm) = RF (q,r,1) sinϕ ,  
where q=cos2ϕ , r=1-m sin2ϕ and RF is the Carlson symmetrised incomplete elliptic integral of the first kind (see s21bbc).

4 References

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

5 Arguments

1: phi double Input
2: dm double Input
On entry: the arguments ϕ and m of the function.
  • 0.0phi π2;
  • dm× sin2(phi) 1.0 ;
  • Only one of sin(phi) and dm may be 1.0.
Note that dm × sin2(phi) = 1.0 is allowable, as long as dm1.0 .
3: fail NagError * Input/Output
The NAG error argument (see Section 7 in the Introduction to the NAG Library CL Interface).

6 Error Indicators and Warnings

Dynamic memory allocation failed.
See Section 3.1.2 in the Introduction to the NAG Library CL Interface for further information.
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.
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.
On entry, phi=value.
Constraint: 0phiπ2.
On failure, the function returns zero.
On entry, phi=value and dm=value; the integral is undefined.
Constraint: dm×sin2(phi)1.0.
On failure, the function returns zero.
On entry, sin(phi)=1 and dm=1.0; the integral is infinite.
On failure, the function returns the largest machine number (see X02ALC).

7 Accuracy

In principle s21bec 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.

8 Parallelism and Performance

Background information to multithreading can be found in the Multithreading documentation.
s21bec is not threaded in any implementation.

9 Further Comments

You should consult the S Chapter Introduction, which shows the relationship between this function and the Carlson definitions of the elliptic integrals. In particular, the relationship between the argument-constraints for both forms becomes clear.
For more information on the algorithm used to compute RF , see the function document for s21bbc.
If you wish to input a value of phi outside the range allowed by this function you should refer to Section 17.4 of Abramowitz and Stegun (1972) for useful identities. For example, F(-ϕ|m)=-F(ϕ|m) and F(sπ±ϕ|m)=2sK(m)±F(ϕ|m) where s is an integer and K(m) is the complete elliptic integral given by s21bhc.
A parameter m>1 can be replaced by one less than unity using F(ϕ|m)=1mF(θ|1m), sinθ=msinϕ.

10 Example

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

10.1 Program Text

Program Text (s21bece.c)

10.2 Program Data


10.3 Program Results

Program Results (s21bece.r)
GnuplotProduced by GNUPLOT 5.4 patchlevel 6 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 0 2 4 6 8 10 gnuplot_plot_1 f m Example Program Classical (Legendre) Form of the Incomplete Elliptic Integral of the First Kind