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.

Factor of automorphy

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

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

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

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

Some facts about factors of automorphy:

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

The case of $G$ a subgroup of $SL_{2}(\mathbb {R} )$, acting on the upper half-plane, is treated in the article on automorphic factors. In particular, automorphic functions for the modular group $G=SL_{2}(\mathbb {Z} )$ are called modular functions.

References

• 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, JFM 55.0810.04
• Fricke, Robert; Klein, Felix (1897), Vorlesungen über die Theorie der automorphen Functionen (in German), vol. I. Die gruppentheoretischen Grundlagen., Leipzig: B. G. Teubner, 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., JFM 32.0430.01