Automorphic function

In mathematics, an automorphic function is a function on a space that is invariant under the action of some group, in other words a function on the quotient space. Often the space is a complex manifold and the group is a discrete group.

In mathematics, the notion of factor of automorphy arises for a group acting on a complex-analytic manifold. Suppose a group ${\displaystyle G}${\displaystyle G} acts on a complex-analytic manifold ${\displaystyle X}${\displaystyle X}. Then, ${\displaystyle G}${\displaystyle G} also acts on the space of holomorphic functions from ${\displaystyle X}${\displaystyle X} to the complex numbers. A function ${\displaystyle f}${\displaystyle f} is termed an automorphic form if the following holds:

${\displaystyle f(g.x)=j_{g}(x)f(x)}${\displaystyle f(g.x)=j_{g}(x)f(x)}

where ${\displaystyle j_{g}(x)}${\displaystyle j_{g}(x)} is an everywhere nonzero holomorphic function. Equivalently, an automorphic form is a function whose divisor is invariant under the action of ${\displaystyle G}${\displaystyle G}.

The factor of automorphy for the automorphic form ${\displaystyle f}${\displaystyle f} is the function ${\displaystyle j}${\displaystyle j}. An automorphic function is an automorphic form for which ${\displaystyle j}${\displaystyle j} is the identity.

Some facts about factors of automorphy:

• Every factor of automorphy is a cocycle for the action of ${\displaystyle G}${\displaystyle G} on the multiplicative group of everywhere nonzero holomorphic functions.
• The factor of automorphy is a coboundary if and only if it arises from an everywhere nonzero automorphic form.
• For a given factor of automorphy, the space of automorphic forms is a vector space.
• The pointwise product of two automorphic forms is an automorphic form corresponding to the product of the corresponding factors of automorphy.

Relation between factors of automorphy and other notions:

• Let ${\displaystyle \Gamma }${\displaystyle \Gamma } be a lattice in a Lie group ${\displaystyle G}${\displaystyle G}. Then, a factor of automorphy for ${\displaystyle \Gamma }${\displaystyle \Gamma } corresponds to a line bundle on the quotient group ${\displaystyle G/\Gamma }${\displaystyle G/\Gamma }. Further, the automorphic forms for a given factor of automorphy correspond to sections of the corresponding line bundle.

The specific case of ${\displaystyle \Gamma }${\displaystyle \Gamma } a subgroup of SL(2, R), acting on the upper half-plane, is treated in the article on automorphic factors.

• Kleinian group – Discrete group of Möbius transformations
• Elliptic modular function – Modular function in mathematicsPages displaying short descriptions of redirect targets
• Modular function – Analytic function on the upper half-plane with a certain behavior under the modular groupPages displaying short descriptions of redirect targets
• Complex torus

• A.N. Parshin (2001) [1994], "Automorphic Form", Encyclopedia of Mathematics , EMS Press
• Andrianov, A.N.; Parshin, A.N. (2001) [1994], "Automorphic Function", Encyclopedia of Mathematics , EMS Press
• Ford, Lester R. (1929), Automorphic functions, New York, McGraw-Hill, ISBN 978-0-8218-3741-2, JFM 55.0810.04
• Fricke, Robert; Klein, Felix (1897), Vorlesungen über die Theorie der automorphen Functionen. Erster Band; Die gruppentheoretischen Grundlagen. (in German), Leipzig: B. G. Teubner, ISBN 978-1-4297-0551-6, JFM 28.0334.01
• Fricke, Robert; Klein, Felix (1912), Vorlesungen über die Theorie der automorphen Functionen. Zweiter Band: Die funktionentheoretischen Ausführungen und die Anwendungen. 1. Lieferung: Engere Theorie der automorphen Funktionen. (in German), Leipzig: B. G. Teubner., ISBN 978-1-4297-0552-3, JFM 32.0430.01

• Automorphic forms
• Discrete groups
• Types of functions
• Complex manifolds

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• CS1 German-language sources (de)