Английская Википедия:Automorphic function

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Шаблон:Short description 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 <math>G</math> acts on a complex-analytic manifold <math>X</math>. Then, <math>G</math> also acts on the space of holomorphic functions from <math>X</math> to the complex numbers. A function <math>f</math> is termed an automorphic form if the following holds:

<math>f(g.x) = j_g(x)f(x)</math>

where <math>j_g(x)</math> is an everywhere nonzero holomorphic function. Equivalently, an automorphic form is a function whose divisor is invariant under the action of <math>G</math>.

The factor of automorphy for the automorphic form <math>f</math> is the function <math>j</math>. An automorphic function is an automorphic form for which <math>j</math> is the identity.

Some facts about factors of automorphy:

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

The specific case of <math>\Gamma</math> a subgroup of SL(2, R), acting on the upper half-plane, is treated in the article on automorphic factors.

Examples

References