(G,X)-manifold

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In geometry, if X is a manifold with an action of a topological group G by analytical diffeomorphisms, the notion of a (G, X)-structure on a topological space is a way to formalise it being locally isomorphic to X with its G-invariant structure; spaces with a (G, X)-structure are always manifolds and are called (G, X)-manifolds. This notion is often used with G being a Lie group and X a homogeneous space for G. Foundational examples are hyperbolic manifolds and affine manifolds.

Definition and examples

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Formal definition

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Let   be a connected differential manifold and   be a subgroup of the group of diffeomorphisms of   which act analytically in the following sense:

if   and there is a nonempty open subset   such that   are equal when restricted to   then  

(this definition is inspired by the analytic continuation property of analytic diffeomorphisms on an analytic manifold).

A  -structure on a topological space   is a manifold structure on   whose atlas' charts has values in   and transition maps belong to  . This means that there exists:

  • a covering of   by open sets   (i.e.  );
  • open embeddings   called charts;

such that every transition map   is the restriction of a diffeomorphism in  .

Two such structures   are equivalent when they are contained in a maximal one, equivalently when their union is also a   structure (i.e. the maps   and   are restrictions of diffeomorphisms in  ).

Riemannian examples

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If   is a Lie group and   a Riemannian manifold with a faithful action of   by isometries then the action is analytic. Usually one takes   to be the full isometry group of  . Then the category of   manifolds is equivalent to the category of Riemannian manifolds which are locally isometric to   (i.e. every point has a neighbourhood isometric to an open subset of  ).

Often the examples of   are homogeneous under  , for example one can take   with a left-invariant metric. A particularly simple example is   and   the group of euclidean isometries. Then a   manifold is simply a flat manifold.

A particularly interesting example is when   is a Riemannian symmetric space, for example hyperbolic space. The simplest such example is the hyperbolic plane, whose isometry group is isomorphic to  .

Pseudo-Riemannian examples

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When   is Minkowski space and   the Lorentz group the notion of a  -structure is the same as that of a flat Lorentzian manifold.

Other examples

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When   is the affine space and   the group of affine transformations then one gets the notion of an affine manifold.

When   is the n-dimensional real projective space and   one gets the notion of a projective structure.[1]

Developing map and completeness

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Developing map

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Let   be a  -manifold which is connected (as a topological space). The developing map is a map from the universal cover   to   which is only well-defined up to composition by an element of  .

A developing map is defined as follows:[2] fix   and let   be any other point,   a path from   to  , and   (where   is a small enough neighbourhood of  ) a map obtained by composing a chart of   with the projection  . We may use analytic continuation along   to extend   so that its domain includes  . Since   is simply connected the value of   thus obtained does not depend on the original choice of  , and we call the (well-defined) map   a developing map for the  -structure. It depends on the choice of base point and chart, but only up to composition by an element of  .

Monodromy

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Given a developing map  , the monodromy or holonomy[3] of a  -structure is the unique morphism   which satisfies

 .

It depends on the choice of a developing map but only up to an inner automorphism of  .

Complete (G,X)-structures

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A   structure is said to be complete if it has a developing map which is also a covering map (this does not depend on the choice of developing map since they differ by a diffeomorphism). For example, if   is simply connected the structure is complete if and only if the developing map is a diffeomorphism.

Examples

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Riemannian (G,X)-structures

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If   is a Riemannian manifold and   its full group of isometry, then a  -structure is complete if and only if the underlying Riemannian manifold is geodesically complete (equivalently metrically complete). In particular, in this case if the underlying space of a  -manifold is compact then the latter is automatically complete.

In the case where   is the hyperbolic plane the developing map is the same map as given by the Uniformisation Theorem.

Other cases

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In general compactness of the space does not imply completeness of a  -structure. For example, an affine structure on the torus is complete if and only if the monodromy map has its image inside the translations. But there are many affine tori which do not satisfy this condition, for example any quadrilateral with its opposite sides glued by an affine map yields an affine structure on the torus, which is complete if and only if the quadrilateral is a parallelogram.

Interesting examples of complete, noncompact affine manifolds are given by the Margulis spacetimes.

(G,X)-structures as connections

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In the work of Charles Ehresmann  -structures on a manifold   are viewed as flat Ehresmann connections on fiber bundles with fiber   over  , whose monodromy maps lie in  .

Notes

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  1. ^ Dumas, Emily (2009). "Complex projective structures". In Papadopoulos, Athanase (ed.). Handbook of Teichmüller theory, Volume II. European MAth. soc.
  2. ^ Thurston 1997, Chapter 3.4.
  3. ^ Thurston 1997, p. 141.

References

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  • Thurston, William (1997). Three-dimensional geometry and topology. Vol. 1. Princeton University Press.