Modular function

In mathematics, modular functions are certain kinds of mathematical function mapping complex numbers to complex numbers. There are a number of other uses of the term "modular function" as well; see below for details.

Formally, a function j is called modular or a modular function iff it satisfies the following properties:

  1. j is meromorphic in the upper half plane H
  2. For every matrix M in the modular group Γ, j(Mτ) = j(τ)
  3. The Laurent series of j has the form
j(\tau) = \sum_{n=-m}^m a(n) e^{2i\pi n\tau}

It can be shown that every modular function can be expressed as a rational function of Klein's absolute invariant J, and that every rational function of J is a modular function; furthermore, all modular functions are modular forms, although the converse does not hold. If a modular function j is not identically 0, then it can be shown that the number of zeroes of j is equal to the number of poles of j in the closure of the fundamental region RΓ.

Other uses

There are a number of other usages of the term modular function, apart from this classical one; for example, in the theory of Haar measure, there is a function Δ(g) determined by the conjugation action.



This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia article. Browse Wikipedia for more information.