Automorphic factor

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In mathematics, an automorphic factor is a certain type of analytic function, defined on subgroups of SL(2,R), appearing in the theory of modular forms. The general case, for general groups, is reviewed in the article 'factor of automorphy'.


An automorphic factor of weight k is a function

satisfying the four properties given below. Here, the notation and refer to the upper half-plane and the complex plane, respectively. The notation is a subgroup of SL(2,R), such as, for example, a Fuchsian group. An element is a 2x2 matrix

with a, b, c, d real numbers, satisfying adbc=1.

An automorphic factor must satisfy:

1. For a fixed , the function is a holomorphic function of .
2. For all and , one has
for a fixed real number k.
3. For all and , one has
Here, is the fractional linear transform of by .
4.If , then for all and , one has
Here, I denotes the identity matrix.


Every automorphic factor may be written as


The function is called a multiplier system. Clearly,


while, if , then


  • Robert Rankin, Modular Forms and Functions, (1977) Cambridge University Press ISBN 0-521-21212-X. (Chapter 3 is entirely devoted to automorphic factors for the modular group.)