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

π x

. 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

- x

by an integer when

0 \leq t \leq \frac{1}{2}

, it follows from the periodicity and symmetry properties of the functions that

θ_{1} (x, q) = \pm θ_{1} (t, q)

and

θ_{3} (x, q) = \pm θ_{3} (t, q)

. In a region for which the approximation is sufficiently accurate,

θ_{1}

is set equal to the first term (

n = 0

) of the transformed series

θ_{1} (t, q) = 2 \sqrt{\frac{λ}{π}} e^{- λ t^{2}} \sum_{n = 0}^{\infty} {(- 1)}^{n} e^{- λ {(n + \frac{1}{2})}^{2}} \sinh {(2 n + 1) λ t}

and

θ_{3}

is set equal to the first two terms (i.e.,

n \leq 1

) of

θ_{3} (t, q) = \sqrt{\frac{λ}{π}} e^{- λ t^{2}} {1 + 2 \sum_{n = 1}^{\infty} e^{- λ n^{2}} \cosh (2 n λ t)},

where

λ = π^{2} / | \log_{e} q |

. Otherwise, the trigonometric series for

θ_{1} (t, q)

and

θ_{3} (t, q)

are used. For all values of

x

θ_{0}

and

θ_{2}

are computed from the relations

θ_{0} (x, q) = θ_{3} (\frac{1}{2} - | x |, q)

and

θ_{2} (x, q) = θ_{1} (\frac{1}{2} - | x |, q)

4 References

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

5 Arguments

1: $k$ – Integer Input: On entry: denotes the function $θ_{k} (x, q)$ to be evaluated. Note that $k = 4$ is equivalent to $k = 0$ .

Constraint: $0 \leq k \leq 4$ .
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: $0.0 \leq q < 1.0$ .
4: $ifail$ – Integer Input/Output: On entry: ifail must be set to $0$ , $−1$ or $1$ to set behaviour on detection of an error; these values have no effect when no error is detected.
A value of $0$ causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of $−1$ means that an error message is printed while a value of $1$ means that it is not.

If halting is not appropriate, the value $−1$ or $1$ is recommended. If message printing is undesirable, then the value $1$ is recommended. Otherwise, the value $0$ is recommended. 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).

6 Error Indicators and Warnings

If on entry

ifail = 0

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $k = ⟨ value ⟩$ .
Constraint: $k \leq 4$ .

On entry, $k = ⟨ value ⟩$ .
Constraint: $k \geq 0$ .

On entry, $q = ⟨ value ⟩$ .
Constraint: $q < 1.0$ .

On entry, $q = ⟨ value ⟩$ .
Constraint: $q \geq 0.0$ .

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

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.

8 Parallelism and Performance

s21ccf is not threaded in any implementation.

9 Further Comments

None.

10 Example

This example evaluates

θ_{2} (x, q)

x = 0.7

when

q = 0.4

, and prints the results.

s21cc: FL CL CPP AD

NAG FL Interfaces21ccf (jactheta_​real)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
s21ccf (jactheta_real)