# Weierstrass functions

(Redirected from Weierstrass eta 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. The relation between the sigma, zeta, and ${\displaystyle \wp }$ functions is analogous to that between the sine, cotangent, and squared cosecant functions: the logarithmic derivative of the sine is the cotangent, whose derivative is negative the squared cosecant.

## Weierstrass sigma function

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

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

where ${\displaystyle \Lambda ^{*}}$  denotes ${\displaystyle \Lambda -\{0\}}$ . See also fundamental pair of periods.

## Weierstrass zeta function

The Weierstrass zeta function is defined by the sum

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

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

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

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

The derivative of the zeta function is ${\displaystyle -\wp (z)}$ , where ${\displaystyle \wp (z)}$  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

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

This is well-defined, i.e. ${\displaystyle \zeta (z+w;\Lambda )-\zeta (z;\Lambda )}$  only depends on the lattice vector w. The Weierstrass eta function should not be confused with either the Dedekind eta function or the Dirichlet eta function.

## Weierstrass ℘-function

The Weierstrass p-function is related to the zeta function by

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

The Weierstrass ℘-function is an even elliptic function of order N=2 with a double pole at each lattice point and no other poles.