# G-structure

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In differential geometry, a ** G-structure** on an

*n*-manifold

*M*, for a given structure group

^{[1]}

*G*, is a

*G*-subbundle of the tangent frame bundle F

*M*(or GL(

*M*)) of

*M*.

The notion of *G*-structures includes many other structures on manifolds, some of them being defined by tensor fields. For example, for the orthogonal group, an O(*n*)-structure defines a Riemannian metric, and for the special linear group an SL(*n*,**R**)-structure is the same as a volume form. For the trivial group, an {*e*}-structure consists of an absolute parallelism of the manifold.

Several structures on manifolds, such as a complex structure, a symplectic structure, or a Kähler structure, are *G*-structures with an additional integrability condition.

## Contents

## Principal bundles and *G*-structures

Although the theory of principal bundles plays an important role in the study of *G*-structures, the two notions are different. A *G*-structure is a principal subbundle of the tangent frame bundle, but the fact that the *G*-structure bundle *consists of tangent frames* is regarded as part of the data. For example, consider two Riemannian metrics on **R**^{n}. The associated O(*n*)-structures are isomorphic if and only if the metrics are isometric. But, since **R**^{n} is contractible, the underlying O(*n*)-bundles are always going to be isomorphic as principal bundles.

This fundamental difference between the two theories can be captured by giving an additional piece of data on the underlying *G*-bundle of a *G*-structure: the **solder form**. The solder form is what ties the underlying principal bundle of the *G*-structure to the local geometry of the manifold itself by specifying a canonical isomorphism of the tangent bundle of *M* to an associated vector bundle. Although the solder form is not a connection form, it can sometimes be regarded as a precursor to one.

In detail, suppose that *Q* is the principal bundle of a *G*-structure. If *Q* is realized as a reduction of the frame bundle of *M*, then the solder form is given by the pullback of the tautological form of the frame bundle along the inclusion. Abstractly, if one regards *Q* as a principal bundle independently of its realization as a reduction of the frame bundle, then the solder form consists of a representation ρ of *G* on **R**^{n} and an isomorphism of bundles θ : *TM* → *Q* ×_{ρ} **R**^{n}.

## Integrability conditions

Several structures on manifolds, such as a complex structure, a symplectic structure, or a Kähler structure, are *G*-structures with an additional integrability condition. Without the corresponding integrability condition, the structure is instead called an "almost" structure, as in an almost complex structure, an almost symplectic structure, or an almost Kähler structure

Specifically, a symplectic manifold structure is a stronger concept than a *G*-structure for the symplectic group. A symplectic structure on a manifold is a two-form *ω* on *M* that is non-degenerate (which is an -structure, or almost symplectic structure), *together with* the extra condition that d*ω* = 0; this latter is called an integrability condition.

Similarly, foliations correspond to *G*-structures coming from block matrices, together with integrability conditions so that the Frobenius theorem applies.

## Isomorphism of *G*-structures

The set of diffeomorphisms of *M* that preserve a *G*-structure is called the *automorphism group* of that structure. For an O(*n*)-structure they are the group of isometries of the Riemannian metric and for an SL(*n*,**R**)-structure volume preserving maps.

Let *P* be a *G*-structure on a manifold *M*, and *Q* a *G*-structure on a manifold *N*. Then an **isomorphism** of the *G*-structures is a diffeomorphism *f* : *M* → *N* such that the pushforward of linear frames *f*_{*} : *FM* → *FN* restricts to give a mapping of *P* into *Q*. (Note that it is sufficient that *Q* be contained within the image of *f*_{*}.) The *G*-structures *P* and *Q* are **locally isomorphic** if *M* admits a covering by open sets *U* and a family of diffeomorphisms *f*_{U} : *U* → *f*(*U*) ⊂ *N* such that *f*_{U} induces an isomorphism of *P*|_{U} → *Q*|_{f(U)}.

An **automorphism** of a *G*-structure is an isomorphism of a *G*-structure *P* with itself. Automorphisms arise frequently^{[2]} in the study of transformation groups of geometric structures, since many of the important geometric structures on a manifold can be realized as *G*-structures.

A **flat G-structure** is a

*G*-structure

*P*having a global section (

*V*

_{1},...,

*V*

_{n}) consisting of commuting vector fields. A

*G*-structure is

**integrable**(or

*locally flat*) if it is locally isomorphic to a flat

*G*-structure.

A wide class of equivalence problems can be formulated in the language of *G*-structures. For example, a pair of Riemannian manifolds are (locally) equivalent if and only if their bundles of orthonormal frames are (locally) isomorphic *G*-structures. In this view, the general procedure for solving an equivalence problem is to construct a system of invariants for the *G*-structure which are then sufficient to determine whether a pair of *G*-structures are locally isomorphic or not.

## Connections on *G*-structures

Let *Q* be a *G*-structure on *M*. A principal connection on the principal bundle *Q* induces a connection on any associated vector bundle: in particular on the tangent bundle. A linear connection ∇ on *TM* arising in this way is said to be **compatible** with *Q*. Connections compatible with *Q* are also called **adapted connections**.

Concretely speaking, adapted connections can be understood in terms of a moving frame.^{[3]} Suppose that *V*_{i} is a basis of local sections of *TM* (i.e., a frame on *M*) which defines a section of *Q*. Any connection ∇ determines a system of basis-dependent 1-forms ω via

- ∇
_{X}V_{i}= ω_{i}^{j}(X)V_{j}

where, as a matrix of 1-forms, ω ∈ Ω^{1}(M)⊗**gl**(*n*). An adapted connection is one for which ω takes its values in the Lie algebra **g** of *G*.

### Torsion of a *G*-structure

Associated to any *G*-structure is a notion of torsion, related to the torsion of a connection. Note that a given *G*-structure may admit many different compatible connections which in turn can have different torsions, but in spite of this it is possible to give an independent notion of torsion *of the G-structure* as follows.^{[4]}

The difference of two adapted connections is a 1-form on *M* with values in the adjoint bundle Ad_{Q}. That is to say, the space *A*^{Q} of adapted connections is an affine space
for Ω^{1}(Ad_{Q}).

The torsion of an adapted connection defines a map

to 2-forms with coefficients in *TM*. This map is linear; its linearization

is called **the algebraic torsion map**. Given two adapted connections ∇ and ∇′, their torsion tensors *T*_{∇}, *T*_{∇′} differ by τ(∇−∇′). Therefore, the image of *T*_{∇} in coker(τ) is independent from the choice of ∇.

The image of *T*_{∇} in coker(τ) for any adapted connection ∇ is called the **torsion** of the *G*-structure. A *G*-structure is said to be **torsion-free** if its torsion vanishes. This happens precisely when *Q* admits a torsion-free adapted connection.

### Example: Torsion for almost complex structures

An example of a *G*-structure is an almost complex structure, that is, a reduction
of a structure group of an even-dimensional manifold to GL(*n*,**C**). Such a reduction is uniquely determined by a *C*^{∞}-linear endomorphism *J* ∈ End(*TM*) such that *J*^{2} = −1. In this situation, the torsion can be computed explicitly as follows.

An easy dimension count shows that

where Ω^{2,0}(*TM*) is a space of forms *B* ∈ Ω^{2}(*TM*) which satisfy

Therefore, the torsion of an almost complex structure can be considered as an element in
Ω^{2,0}(*TM*). It is easy to check that the torsion of an almost complex structure is equal to its Nijenhuis tensor.

## Higher order *G*-structures

Imposing integrability conditions on a particular *G*-structure (for instance, with the case of a symplectic form) can be dealt with via the process of prolongation. In such cases, the prolonged *G*-structure cannot be identified with a *G*-subbundle of the bundle of linear frames. In many cases, however, the prolongation is a principal bundle in its own right, and its structure group can be identified with a subgroup of a higher-order jet group. In which case, it is called a higher order *G*-structure [Kobayashi]. In general, Cartan's equivalence method applies to such cases.

## See also

## Notes

- ↑ Which is a Lie group mapping to the general linear group . This is often but not always a Lie subgroup; for instance, for a spin structure the map is a covering space onto its image.
- ↑ Kobayashi (1972).
- ↑ Kobayashi (1972) I.4.
- ↑ Gauduchon (1997).

## References

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