# Weierstrass functions

(Redirected from Weierstrass sigma function)

In mathematics, the Weierstrass functions are special functions of a complex variable that are auxiliary to the Weierstrass elliptic function. They are named for Karl Weierstrass.

## Weierstrass sigma-function

The Weierstrass sigma-function associated to a two-dimensional lattice $\Lambda \subset \mathbb{C}$ is defined to be the product

$\sigma (z;\Lambda )=z\prod _{{w\in \Lambda ^{{*}}}}\left(1-{\frac {z}{w}}\right)e^{{z/w+{\frac {1}{2}}(z/w)^{2}}}$

where $\Lambda ^{{*}}$ denotes $\Lambda -\{0\}$.

## Weierstrass zeta-function

The Weierstrass zeta-function is defined by the sum

$\zeta (z;\Lambda )={\frac {\sigma '(z;\Lambda )}{\sigma (z;\Lambda )}}={\frac {1}{z}}+\sum _{{w\in \Lambda ^{{*}}}}\left({\frac {1}{z-w}}+{\frac {1}{w}}+{\frac {z}{w^{2}}}\right).$

Note that the Weierstrass zeta-function is basically the logarithmic derivative of the sigma-function. The zeta-function can be rewritten as:

$\zeta (z;\Lambda )={\frac {1}{z}}-\sum _{{k=1}}^{{\infty }}{\mathcal {G}}_{{2k+2}}(\Lambda )z^{{2k+1}}$

where ${\mathcal {G}}_{{2k+2}}$ is the Eisenstein series of weight 2k + 2.

Also note that the derivative of the zeta-function is $-\wp (z)$, where $\wp (z)$ is the Weierstrass elliptic function

The Weierstrass zeta-function should not be confused with the Riemann zeta-function in number theory.

## Weierstrass eta-function

The Weierstrass eta-function is defined to be

$\eta (w;\Lambda )=\zeta (z+w;\Lambda )-\zeta (z;\Lambda ),{\mbox{ for any }}z\in \mathbb{C}$

It can be proved that this is well-defined, i.e. $\zeta (z+w;\Lambda )-\zeta (z;\Lambda )$ only depends on w. The Weierstrass eta-function should not be confused with the Dedekind eta-function.

## Weierstrass p-function

The Weierstrass p-function is defined to be

$\wp (z;\Lambda )=-\zeta '(z;\Lambda ),{\mbox{ for any }}z\in \mathbb{C}$

The Weierstrass p-function is an even elliptic function of order N=2 with a double pole at each lattice and no others.