General Theory
℘ is a meromorphic function in the complex plane with a double pole at each lattice points. It is doubly periodic with periods 1 and τ; this means that ℘ satisfies
The above sum is homogeneous of degree minus two, and if c is any non-zero complex number,
from which we may define the Weierstrass ℘ function for any pair of periods. We also may take the derivative (of course, with respect to z) and obtain a function algebraically related to ℘ by
where and depend only on τ, being modular forms. The equation
defines an elliptic curve, and we see that is a parametrization of that curve.
The totality of meromorphic doubly periodic functions with given periods defines an algebraic function field, associated to that curve. It can be shown that this field is
so that all such functions are rational functions in the Weierstrass function and its derivative.
We can also wrap a single period parallelogram into a torus, or donut-shaped Riemann surface, and regard the elliptic functions associated to a given pair of periods to be functions defined on that Riemann surface.
The roots e1, e2, and e3 of the equation depend on τ and can be expressed in terms of theta functions; we have
Since and we have these in terms of theta functions also.
We may also express ℘ in terms of theta functions; because these converge very rapidly, this is a more expeditious way of computing ℘ than the series we used to define it.
The function ℘ has two zeros (modulo periods) and the function ℘′ has three. The zeros of ℘′ are easy to find: since ℘′ is an odd function they must be at the half-period points. On the other hand it is very difficult to express the zeros of ℘ by closed formula, except for special values of the modulus (e.g. when the period lattice is the Gaussian integers). An expression was found, by Zagier and Eichler.
The Weierstrass theory also includes the Weierstrass zeta function, which is an indefinite integral of ℘ and not doubly periodic, and a theta function called the Weierstrass sigma function, of which his zeta-function is the log-derivative. The sigma-function has zeros at all the period points (only), and can be expressed in terms of Jacobi's functions. This gives one way to convert between Weierstrass and Jacobi notations.
The Weierstrass sigma-function is an entire function; it played the role of 'typical' function in a theory of random entire functions of J. E. Littlewood.
Read more about this topic: Weierstrass's Elliptic Functions
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