Neville theta functions

For other θ functions, see Theta function (disambiguation).

In mathematics, the Neville theta functions, named after Eric Harold Neville, are defined as follows:

$$\theta _{c}(z,m)={\frac {{\sqrt {2\pi }}\,q(m)^{1/4}}{m^{1/4}{\sqrt {K(m)}}}}\,\,\sum _{k=0}^{\infty }(q(m))^{k(k+1)}\cos \left({\frac {(2k+1)\pi z}{2K(m)}}\right)$$

$$\theta _{d}(z,m)={\frac {\sqrt {2\pi }}{2{\sqrt {K(m)}}}}\,\,\left(1+2\,\sum _{k=1}^{\infty }(q(m))^{k^{2}}\cos \left({\frac {\pi zk}{K(m)}}\right)\right)$$

$$\theta _{n}(z,m)={\frac {\sqrt {2\pi }}{2(1-m)^{1/4}{\sqrt {K(m)}}}}\,\,\left(1+2\sum _{k=1}^{\infty }(-1)^{k}(q(m))^{k^{2}}\cos \left({\frac {\pi zk}{K(m)}}\right)\right)$$

$$\theta _{s}(z,m)={\frac {{\sqrt {2\pi }}\,q(m)^{1/4}}{m^{1/4}(1-m)^{1/4}{\sqrt {K(m)}}}}\,\,\sum _{k=0}^{\infty }(-1)^{k}(q(m))^{k(k+1)}\sin \left({\frac {(2k+1)\pi z}{2K(m)}}\right)$$

where: K(m) is the complete elliptic integral of the first kind, $K'(m)=K(1-m)$, and $q(m)=e^{-\pi K'(m)/K(m)}$ is the elliptic nome.

Note that the functions θ_p(z,m) are sometimes defined in terms of the nome q(m) and written θ_p(z,q) (e.g. NIST). The functions may also be written in terms of the τ parameter θ_p(z|τ) where $q=e^{i\pi \tau }$.

Relationship to other functions

The Neville theta functions may be expressed in terms of the Jacobi theta functions

$$\theta _{s}(z|\tau )=\theta _{3}^{2}(0|\tau )\theta _{1}(z'|\tau )/\theta '_{1}(0|\tau )$$ $$\theta _{c}(z|\tau )=\theta _{2}(z'|\tau )/\theta _{2}(0|\tau )$$ $$\theta _{n}(z|\tau )=\theta _{4}(z'|\tau )/\theta _{4}(0|\tau )$$ $$\theta _{d}(z|\tau )=\theta _{3}(z'|\tau )/\theta _{3}(0|\tau )$$

where $z'=z/\theta _{3}^{2}(0|\tau )$.

The Neville theta functions are related to the Jacobi elliptic functions. If pq(u,m) is a Jacobi elliptic function (p and q are one of s,c,n,d), then

$$\operatorname {pq} (u,m)={\frac {\theta _{p}(u,m)}{\theta _{q}(u,m)}}.$$

Examples

• $\theta _{c}(2.5,0.3)\approx -0.65900466676738154967$
• $\theta _{d}(2.5,0.3)\approx 0.95182196661267561994$
• $\theta _{n}(2.5,0.3)\approx 1.0526693354651613637$
• $\theta _{s}(2.5,0.3)\approx 0.82086879524530400536$

Symmetry

• $\theta _{c}(z,m)=\theta _{c}(-z,m)$
• $\theta _{d}(z,m)=\theta _{d}(-z,m)$
• $\theta _{n}(z,m)=\theta _{n}(-z,m)$
• $\theta _{s}(z,m)=-\theta _{s}(-z,m)$

Complex 3D plots

References

• Weisstein, Eric W. "Neville Theta Functions". MathWorld.