NAG Library Routine Document

s21ccf (jactheta_real)


s21ccf returns the value of one of the Jacobian theta functions θ0x,q, θ1x,q, θ2x,q, θ3x,q or θ4x,q for a real argument x and non-negative q<1, via the function name.


Fortran Interface
Function s21ccf ( k, x, q, ifail)
Real (Kind=nag_wp):: s21ccf
Integer, Intent (In):: k
Integer, Intent (Inout):: ifail
Real (Kind=nag_wp), Intent (In):: x, q
C Header Interface
#include <nagmk26.h>
double  s21ccf_ (const Integer *k, const double *x, const double *q, Integer *ifail)


s21ccf evaluates an approximation to the Jacobian theta functions θ0x,q, θ1x,q, θ2x,q, θ3x,q and θ4x,q given by
θ0x,q = 1+2n=1-1nqn2cos2nπx, θ1x,q = 2n=0-1nq n+12 2sin2n+1πx, θ2x,q = 2n=0q n+12 2cos2n+1πx, θ3x,q = 1+2n=1qn2cos2nπx, θ4x,q = θ0x,q,  
where x and q (the nome) are real with 0q<1.
These functions are important in practice because every one of the Jacobian elliptic functions (see s21cbf) can be expressed as the ratio of two Jacobian theta functions (see Whittaker and Watson (1990)). There is also a bewildering variety of notations used in the literature to define them. Some authors (e.g., Section 16.27 of Abramowitz and Stegun (1972)) define the argument in the trigonometric terms to be x instead of πx. This can often lead to confusion, so great care must therefore be exercised when consulting the literature. Further details (including various relations and identities) can be found in the references.
s21ccf is based on a truncated series approach. If t differs from x or -x by an integer when 0t12 , it follows from the periodicity and symmetry properties of the functions that θ1x,q=±θ1t,q and θ3x,q=±θ3t,q. In a region for which the approximation is sufficiently accurate, θ1 is set equal to the first term (n=0) of the transformed series
θ1t,q=2λπe-λt2n=0-1ne-λ n+12 2sinh2n+1λt  
and θ3 is set equal to the first two terms (i.e., n1) of
θ3t,q=λπe-λt2 1+2n=1e-λn2cosh2nλt ,  
where λ= π2/ logeq . Otherwise, the trigonometric series for θ1t,q and θ3t,q are used. For all values of x, θ0 and θ2 are computed from the relations θ0x,q=θ312-x,q and θ2x,q=θ112-x,q.


Abramowitz M and Stegun I A (1972) Handbook of Mathematical Functions (3rd Edition) Dover Publications
Byrd P F and Friedman M D (1971) Handbook of Elliptic Integrals for Engineers and Scientists pp. 315–320 (2nd Edition) Springer–Verlag
Magnus W, Oberhettinger F and Soni R P (1966) Formulas and Theorems for the Special Functions of Mathematical Physics 371–377 Springer–Verlag
Tølke F (1966) Praktische Funktionenlehre (Bd. II) 1–38 Springer–Verlag
Whittaker E T and Watson G N (1990) A Course in Modern Analysis (4th Edition) Cambridge University Press


1:     k – IntegerInput
On entry: denotes the function θkx,q to be evaluated. Note that k=4 is equivalent to k=0.
Constraint: 0k4.
2:     x – Real (Kind=nag_wp)Input
On entry: the argument x of the function.
3:     q – Real (Kind=nag_wp)Input
On entry: the argument q of the function.
Constraint: 0.0q<1.0.
4:     ifail – IntegerInput/Output
On entry: ifail must be set to 0, -1 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 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 -1 or 1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

Error Indicators and Warnings

If on entry 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:
On entry, k=value.
Constraint: k4.
On entry, k=value.
Constraint: k0.
On entry, q=value.
Constraint: q<1.0.
On entry, q=value.
Constraint: q0.0.
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
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.
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.


In principle the routine is capable of achieving full relative precision in the computed values. However, the accuracy obtainable in practice depends on the accuracy of the standard elementary functions such as sin and cos.

Parallelism and Performance

s21ccf is not threaded in any implementation.

Further Comments



This example evaluates θ2x,q at x=0.7 when q=0.4, and prints the results.

Program Text

Program Text (s21ccfe.f90)

Program Data

Program Data (s21ccfe.d)

Program Results

Program Results (s21ccfe.r)