Symmetric space
A general name given to various types of spaces in differential geometry.
1) A manifold with an affine connection is called a locally symmetric affine space if the torsion tensor and the covariant derivative of the curvature tensor vanish identically.
2) A (pseudo-) Riemannian manifold is called a locally symmetric (pseudo-) Riemannian space if the covariant derivative of its curvature tensor with respect to the Levi-Civita connection vanishes identically.
3) A pseudo-Riemannian manifold (respectively, a manifold with an affine connection) is called a globally symmetric pseudo-Riemannian (affine) space if one can assign to every point
an isometry (affine transformation)
of
such that
and
is an isolated fixed point of
.
4) Let be a connected Lie group, let
be an involutive automorphism
, let
be the closed subgroup of all
-fixed points, let
be the component of the identity in
, and let
be a closed subgroup of
such that
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Then the homogeneous space is called a symmetric homogeneous space.
5) A symmetric space in the sense of Loos (a Loos symmetric space) is a manifold endowed with a multiplication
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satisfying the following four conditions:
a) ;
b) ;
c) ;
d) every point has a neighbourhood
such that
implies
for all
.
Any globally symmetric affine (pseudo-Riemannian) space is a locally symmetric affine (pseudo-Riemannian) space and a homogeneous symmetric space. Any homogeneous symmetric space is a globally symmetric affine space and a Loos symmetric space. Every connected Loos symmetric space is a homogeneous symmetric space.
Let be a connected Loos symmetric space, and hence a homogeneous space:
. Then
can be equipped with a torsion-free invariant affine connection with the following properties:
) the covariant derivative of the curvature tensor vanishes;
) every geodesic
is a trajectory of some one-parameter subgroup
of
, and parallel translation of vectors along
coincides with their translation by means of
; and
) the geodesics are closed under multiplication (they are called one-dimensional subspaces). Similarly one can introduce the concept of an arbitrary subspace of
, namely, a manifold
of
which is closed under multiplication and which is a symmetric space under the induced multiplication. A closed subset
of
which is stable under multiplication is a subspace. The analogue of the Lie algebra for a symmetric space
is defined as follows. Let
and
be the Lie algebras of the groups
and
, respectively, and let
(the differential at the unit), where
is the involutive automorphism defining the symmetric homogeneous space
. The eigenvectors of the space endomorphism
corresponding to the eigenvalue
form a subspace
such that
is the direct sum of the subspaces
and
, and
can be identified with the tangent space of
at the point
. If one defines a trilinear composition on the vector space
by
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where is the curvature tensor, then
becomes a Lie ternary system. If
is a subspace of
passing through the point 0, then the tangent space of
at 0 is a subsystem of
and conversely.
If is a Loos symmetric space, then so is the product
. Let
be a subspace of
defining an equivalence relation on
. Then
is called a congruence. This concept is used in the construction of a theory of coverings for symmetric spaces. Two points
are said to commute if
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The centre of
with respect to a point
is defined to be the set of all points of
which commute with 0.
is a closed subspace of
which can be equipped with an Abelian group structure. Let
be a simply-connected symmetric space. Then the search for symmetric spaces for which
is a covering space reduces to the classification of discrete subgroups of
.
In the theory of symmetric spaces, considerable attention is devoted to classification problems (see ). Let be a locally symmetric Riemannian space. Then
is called reducible if, in some coordinate system, its fundamental quadratic form can be written as
![]() |
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Otherwise the space is called irreducible. E. Cartan has shown that the study of all irreducible locally symmetric Riemannian spaces reduces to the classification of involutive automorphisms of real compact Lie algebras, which he accomplished. At the same time he solved the local classification problem for symmetric homogeneous spaces whose fundamental groups are simple and compact. A classification of symmetric homogeneous spaces with simple non-compact fundamental groups has been obtained (see , [3], [5]).
References
[1] | P.A. Shirokov, "Selected works on geometry" , Kazan' (1966) (In Russian) |
[2a] | E. Cartan, "Sur une classe rémarkable d'espaces de Riemann" Bull. Soc. Math. France , 54 (1926) pp. 214–264 |
[2b] | E. Cartan, "Sur une classe rémarkable d'espaces de Riemann" Bull. Soc. Math. France , 55 (1927) pp. 114–134 |
[3] | M. Berger, "Les espaces symmétriques noncompacts" Ann. Sci. École Norm. Sup. , 74 (1957) pp. 85–177 |
[4] | O. Loos, "Symmetric spaces" , 1–2 , Benjamin (1969) |
[5] | S. Helgason, "Differential geometry, Lie groups, and symmetric spaces" , Acad. Press (1978) |
Comments
Let be a globally symmetric Riemannian space,
the connected component of the group of isometries of
and
the isotropy subgroup of
of some point of
. Then definitions can be given for
being of compact, non-compact or Euclidean type in terms of the corresponding pair of Lie algebras
. In particular, if
is of the non-compact type, then
has a Cartan decomposition
, see [5].
References
[a1] | A.L. Besse, "Einstein manifolds" , Springer (1987) |
[a2] | B.A. [B.A. Rozenfel'd] Rosenfel'd, "A history of non-euclidean geometry" , Springer (1988) (Translated from Russian) |
Symmetric space. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Symmetric_space&oldid=28422