Jacobian_theta

Definition:

The y = jacobian_theta(k, x, q) computes the value of one of the Jacobian theta functions $\theta _0 (x, q)$ , $\theta _1 (x, q)$ , $\theta _2 (x, q)$ , $\theta _3 (x, q)$ or $\theta _4 (x, q)$ for a real argument x and non-negative q ≤ 1.

The routine evaluates an approximation to the Jacobian theta functions $\theta _0 (x, q)$ , $\theta _1 (x, q)$ , $\theta _2 (x, q)$ , $\theta _3 (x, q)$ and $\theta _4 (x, q)$ given by

$\theta _0\left( x,q\right) =1+2\sum_{n=1}^\infty \left( -1\right) ^nq^{n^2}\cos (2n\pi x)$ ,

$\theta _1\left( x,q\right) =2\sum_{n=0}^\infty \left( -1\right)^nq^{(n+0.5)^2}\sin((2n+1)\pi x)$ ,

$\theta _2\left( x,q\right) =2\sum_{n=0}^\infty q^{(n+0.5)^2}\cos ((2n+1)\pi x)$ ,

$\theta _3\left( x,q\right) =1+2\sum_{n=1}^\infty q^{n^2}\cos (2n\pi x)$ ,

$\theta _4\left( x,q\right) =\theta _0\left( x,q\right)$ ,

where x and q are real with 0 ≤ q ≤ 1. Note that $\theta _1 (x-0.5, 1)$ is undefined if $(x-0.5)$ is an integer, as is $\theta _2 (x, 1)$ if x is an integer. Otherwise, $\theta _i (x, 1)=0$ , for $i=0, 1, ..., 4$ .

For more information please refer to the s21ccc function in the NAG document.

Parameters:

k (input, integer): The function $\theta _k$ (x,q) to be evaluated. Note that k=4 is equivalent to k=0.; Constraint: 0 ≤ k ≤ 4.
x (input, double): The argument x of the function.; Constraints: x must not be an integer when q=1.0 and k=2; (x-0.5) must not be an integer when q=1.0 and k=1.
q (input, double): The argument q of the function.; Constraint: 0.0 ≤ q ≤ 1.0.
y (output, double): The return value of one of the Jacobian theta functions.

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