Symmetric space

In differential geometry, representation theory and harmonic analysis, a symmetric space is a smooth manifold whose group of symmetries contains an "inversion symmetry" about every point. There are two ways to make this precise. In Riemannian geometry, the inversions are geodesic symmetries, and these are required to be isometries, leading to the notion of a Riemannian symmetric space. More generally a symmetric space is a homogeneous space "G"/"H" for a Lie group "G" such that the stabilizer "H" of a point is an open subgroup of the fixed point set of an involution of "G". This definition includes (globally) Riemannian symmetric spaces and pseudo-Riemannian symmetric spaces as special cases.

Riemannian symmetric spaces arise in a wide variety of situations in both mathematics and physics. They were first studied extensively and classified by Élie Cartan. More generally, classifications of irreducible and semisimple symmetric spaces have been given by Marcel Berger. They are important in representation theory and harmonic analysis as well as differential geometry.

Definition using geodesic symmetries

Let "M" be a connected Riemannian manifold and "p" a point of "M". A map "f" defined on a neighborhood of "p" is said to be a geodesic symmetry, if it fixes the point "p" and reverses geodesics through that point. It follows that the derivative of the map at "p" is minus the identity map on the tangent space of "p". On a general Riemannian manifold, "f" need not be isometric, nor can it be extended, in general, from a neighbourhood of "p" to all of "M".

"M" is said to be locally Riemannian symmetric if its geodesic symmetries are in fact isometric, and (globally) Riemannian symmetric if in addition its geodesic symmetries are defined on all of "M".

Basic properties

The Cartan-Ambrose-Hicks Theorem imples that "M" is locally Riemannian symmetric if and only if its curvature tensor is covariantly constant, and furthermore that any simply connected, complete locally Riemannian symmetric space is actually Riemannian symmetric.

Any Riemannian symmetric space "M" is complete and Riemannian homogeneous (meaning that the isometry group of "M" acts transitively on "M"). In fact, already the identity component of the isometry group acts transitively on "M" (because "M" is connected). Locally Riemannian symmetric spaces that are not Riemannian symmetric may be constructed as quotients of Riemannian symmetric spaces by discrete groups of isometries with no fixed points, and as open subsets of (locally) Riemannian symmetric spaces.

Examples

Basic examples of Riemannian symmetric spaces are Euclidean space, spheres, projective spaces, and hyperbolic spaces, each with their standard Riemannian metrics. More examples are provided by compact, semi-simple Lie groups equipped with a bi-invariant Riemannian metric.

Any compact Riemann surface of genus greater than 1 (with its usual metric of constant curvature −1) is a locally symmetric space but not a symmetric space.

General definition

Let "G" be a connected Lie group. Then a symmetric space for "G" is a homogeneous space "G"/"H" where the stabilizer "H" of a typical point is an open subgroup of the fixed point set of an involution "σ" of "G". Thus "σ" is an automorphism of "G" with "σ"² = id_"G" and "H" is a open subgroup of the set: $G^sigma=\{\; gin\; G:\; sigma(g)\; =\; g\}.$Because "H" is open, it is a union of components of "G"^"σ" (including, of course, the identity component).

As an automorphism of "G", "σ" fixes the identity element, and hence, by differentiating at the identity, it induces an automorphism of the Lie algebra $mathfrak\; g$ of "G", also denoted by "σ", whose square is the identity. It follows that the eigenvalues of "σ" are ±1. The +1 eigenspace is the Lie algebra $mathfrak\; h$ of "H" (since this is the Lie algebra of "G"^"σ"), and the -1 eigenspace will be denoted $mathfrak\; m$. Since "σ" is an automorphism of $mathfrak\; g$, this gives a direct sum decomposition:$mathfrak\; g\; =\; mathfrak\; hoplusmathfrak\; m$with:$[mathfrak\; h,mathfrak\; h]\; subset\; mathfrak\; h,;\; [mathfrak\; h,mathfrak\; m]\; subset\; mathfrak\; m,;\; [mathfrak\; m,mathfrak\; m]\; subset\; mathfrak\; h.$The first condition is automatic for any homogeneous space: it just says the infinitesimal stabilizer $mathfrak\; h$ is a Lie subalgebra of $mathfrak\; g$. The second condition means that $mathfrak\; m$ is an $mathfrak\; h$-invariant complement to $mathfrak\; h$ in $mathfrak\; g$. Thus any symmetric space is a reductive homogeneous space, but there are many reductive homogeneous spaces which are not symmetric spaces. The key feature of symmetric spaces is the third condition that $mathfrak\; m$ brackets into $mathfrak\; h$.

Conversely, given any Lie algebra $mathfrak\; g$ with a direct sum decomposition satisfying these three conditions, the linear map "σ", equal to the identity on $mathfrak\; h$ and minus the identity on $mathfrak\; m$, is an involutive automorphism.

Riemannian symmetric spaces are symmetric spaces

If "M" is a Riemannian symmetric space, the identity component "G" of the isometry group of "M" is a Lie group acting transitively on "M" ("M" is Riemannian homogeneous). Therefore, if we fix some point "p" of "M", "M" is diffeomorphic to the quotient "G/K", where "K" denotes the isotropy group of the action of "G" on "M" at "p". By differentiating the action at "p" we obtain an isometric action of "K" on T_"p""M". This action is faithful (e.g., by a theorem of Kostant, any isometry in the identity component is determined by its 1-jet at any point) and so "K" is a subgroup of the orthogonal group of T_"p""M", hence compact. Moreover, if we denote by "s"_"p": M → M the geodesic symmetry of "M" at "p", the
$sigma:\; G\; o\; G,\; h\; mapsto\; s\_p\; circ\; h\; circ\; s\_p$is an involutive Lie group automorphism such that the isotropy group "K" is contained between the fixed point group of "σ" and its identity component (hence an open subgroup).

To summarize, "M" is a symmetric space "G"/"K" with a compact isotropy group "K". Conversely, symmetric spaces with compact isotropy group are Riemannian symmetric spaces, although not necessarily in a unique way. To obtain a Riemannian symmetric space structure we need to fix a "K"-invariant inner product on the tangent space to "G"/"K" at the identity coset "eK": such an inner product always exists by averaging, since "K" is compact, and by acting with "G", we obtain a "G"-invariant Riemannian metric "g" on "G"/"K".

To show that "G"/"K" is Riemannian symmetric, consider any point "p" = "hK" (a coset of "K", where "h" ∈ "G") and define:$s\_p:\; M\; o\; M,\; h\text{'}K\; mapsto\; h\; sigma(h^\{-1\}k\text{'})K$where "σ" is the involution of "G" fixing "K". Then one can check that "s"_"p" is an isometry with (clearly) "s"_"p"("p") = "p" and (by differentiating) d"s"_"p" equal to minus the identity on T_"p""M". Thus "s"_"p" is a geodesic symmetry and, since "p" was arbitrary, "M" is a Riemannian symmetric space.

If one starts with a Riemannian symmetric space "M", and then performs these two constructions in sequence, then the Riemannian symmetric space yielded is isometric to the original one. This shows that the "algebraic data" ("G","K","σ","g") completely describe the structure of "M".

Classification of Riemannian symmetric spaces

The algebraic description of Riemannian symmetric spaces enabled Élie Cartan to obtain a complete classification of them in 1926.

For a given Riemannian symmetric space "M" let ("G","K","σ","g") the algebraic data associated to it. To classify possibly isometry classes of "M", first note that the universal cover of a Riemannian symmetric space is again Riemannian symmetric, and the covering map is described by dividing the connected isometry group "G" of the covering by a subgroup of its center. Therefore we may suppose without loss of generality that "M" is simply connected. (This implies "K" is connected by the long exact sequence of a fibration, because "G" is connected by assumption.)

Classification scheme

A simply connected Riemannian symmetric space is said to be irreducible if it is not the product of two or more Riemannian symmetric spaces. It can then be shown that any simply connected Riemannian symmetric space is a Riemannian product of irreducible ones. Therefore we may further restrict ourselves to classifying the irreducible, simply connected Riemannian symmetric spaces.

The next step is to show that any irreducible, simply connected Riemannian symmetric space "M" is of either of the following three types:

1. Euclidean type: "M" has vanishing curvature, and is therefore isometric to a Euclidean space.

2. Compact type: "M" has nonnegative (but not identically zero) sectional curvature.

3. Non-compact type: "M" has nonpositive (but not identically zero) sectional curvature.

A more refined invariant is the rank, which is the maximum dimension of a subspace of the tangent space (to any point) on which the curvature is identically zero. For spaces of compact or noncompact type, the rank is at least one, with equality if the sectional curvature is positive or negative respectively. The spaces of Euclidean type have rank equal to their dimension and are isometric to a Euclidean space of that dimension. Therefore it remains to classify the irreducible, simply connected Riemannian symmetric spaces of compact and non-compact type. In both cases there are two classes.

A. "G" is a (real) simple Lie group;

B. "G" is either the product of a compact simple Lie group with itself (compact type), or a complexification of such a Lie group (non-compact type).

The examples in class B are completely described by the classification of simple Lie groups. For compact type, "M" is a compact simply connected simple Lie group, "G" is "M"×"M" and "K" is the diagonal subgroup. For non-compact type, "G" is a simply connected complex simple Lie group and "K" is its maximal compact subgroup. In both cases, the rank is the rank of "G".

The compact simply connected Lie groups are the universal covers of the classical Lie groups $mathrm\{SO\}(n)$, $mathrm\{SU\}(n)$, $mathrm\{Sp\}(n)$ and the five exceptional Lie groups "E₆", "E₇", "E₈", "F₄", "G₂".

The examples of class A are completely described by the classification of noncompact simply connected real simple Lie groups. For non-compact type, "G" is such a group and "K" is its maximal compact subgroup. Each such example has a corresponding example of compact type, by considering a maximal compact subgroup of the complexification of "G" which contains "K". More directly, the examples of compact type are classified by involutive automorphisms of compact simply connected simple Lie groups "G" (up to conjugation). Such involutions extend to involutions of the complexification of "G", and these in turn classify non-compact real forms of "G".

In both class A and class B there is thus a correspondence between symmetric spaces of compact type and non-compact type. This is known as duality for Riemannian symmetric spaces.

Classification result

Specializing to the Riemannian symmetric spaces of class A and compact type, Cartan found that there are the following seven infinite series and twelve exceptional Riemannian symmetric spaces "G"/"K". They are here given in terms of "G" and "K", together with a geometric interpretation, if readily available. The labelling of these spaces is the one given by Cartan.

Weakly symmetric Riemannian spaces

In the 1950s Atle Selberg extended Cartan's definition of symmetric space to that of weakly symmetric Riemannian space, or in current terminology weakly symmetric space. These are defined as Riemannian manifolds "M" with a transitive connected Lie group of isometries "G" and an isometry σ normalising "G" such that given "x", "y" in "M" there is an isometry "s" in "G" such that "sx" = σ"y" and "sy" = σ"x". (Selberg's assumption that "s"² should be an element of "G" was later shown to be unnecessary by Vinberg] .) Selberg proved that weakly symmetric spaces give rise to Gelfand pairs, so that in particular the unitary representation of "G" on "L"²("M") is multiplicity free.

Selberg's definition can also be phrased equivalently in terms of a generalization of geodesic symmetry. It is required that for every point "x" in "M" and tangent vector "X" at "x", there is an isometry "s" of "M", depending on "x" and "X", such that

*"s" fixes "x";
*the derivative of "s" at "x" sends "X" to –"X".

When "s" is independent of "X", "M" is a symmetric space. An account of weakly symmetric spaces and their classification by Akhiezer and Vinberg, based on the classification of periodic automorphisms of complex semisimple Lie algebras, is given in harvtxt|Wolf|2007.

Applications and special cases

ymmetric spaces and holonomy

If the identity component of the holonomy group of a Riemannian manifold at a point acts irreducibly on the tangent space, then either the manifold is a locally Riemannian symmetric space, or it is in one of 7 families.

Hermitian symmetric spaces

A Riemannian symmetric space which is additionally equipped with a parallel complex structure compatible with the Riemannian metric is called a Hermitian symmetric space. Some examples are complex vector spaces and complex projective spaces, both with their usual Riemannian metric, and the complex unit balls with suitable metrics so that they become complete and Riemannian symmetric.

An irreducible symmetric space "G"/"K" is Hermitian if and only if "K" contains a central circle. A quarter turn by this circle acts as multiplication by "i" on the tangent space at the identity coset. Thus the Hermitian symmetric spaces are easily read off of the classification. In both the compact and the non-compact cases it turns out that there are four infinite series, namely AIII, BDI with "p=2", DIII and CI, and two exceptional spaces, namely EIII and EVII. The non-compact Hermitian symmetric spaces can be realized as bounded symmetric domains in complex vector spaces.

Quaternion-Kähler symmetric spaces

A Riemannian symmetric space which is additionally equipped with a parallel subbundle of End(T"M") isomorphic to the imaginary quaternions at each point, and compatible with the Riemannian metric, is called Quaternion-Kähler symmetric space.

An irreducible symmetric space "G"/"K" is quaternion-Kähler if and only if isotropy representation of "K" contains an Sp(1) summand acting like the unit quaternions on a quaternionic vector space. Thus the quaternion-Kähler symmetric spaces are easily read off from the classification. In both the compact and the non-compact cases it turns out that there is exactly one for each complex simple Lie group, namely AI with "p"=2 or "q"=2 (these are isomorphic), BDI with "p"=4 or "q"=4, CII with "p"=1 or "q"=1, EII, EVI, EIX, FI and G.

References

*citation|first=D. N.|last=Akhiezer|first2=E. B.|last2=Vinberg|title=Weakly symmetric spaces and spherical varieties|journal=Transf. Groups|volume=4|year=1999|pages=3–24
*citation|first= E. P. |last=van den Ban|first2= M. |last2=Flensted-Jensen|first3= H.|last3= Schlichtkrull|title=Harmonic analysis on semisimple symmetric spaces: A survey of some general results|series= in Representation Theory and Automorphic Forms: Instructional Conference, International Centre for Mathematical Sciences, March 1996, Edinburgh, Scotland| publisher=American Mathematical Scoiety|year= 1997|id= ISBN 978-0821806098
*citation| first=Marcel|last= Berger|authorlink=Marcel Berger|title=Les espaces symmétriques noncompacts|journal= Annales scientifiques de l’École Normale Superieure|volume=74|year=1957|pages=85–177|url=http://www.numdam.org/item?id=ASENS_1957_3_74_2_85_0
* Contains a compact introduction and lots of tables.
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* citation|first=Mogens|last= Flensted-Jensen|title= Analysis on Non-Riemannian Symmetric Spaces|series= CBMS Regional Conference|publisher= Americal Mathematical Society| year= 1986|id=ISBN 978-0821807118
*citation|first=Sigurdur|last=Helgason|title=Differential geometry, Lie groups and symmetric spaces|year=1978|publisher=Academic Press|id=ISBN 0-12-338460-5 The standard book on Riemannian symmetric spaces.
*citation|first=Sigurdur|last=Helgason|title=Groups and Geometric Analysis: Integral Geometry, Invariant Differential Operators, and Spherical Functions|year=1984|publisher=Academic Press|id=ISBN 0-12-338301-3
* Chapter XI contains a good introduction to Riemannian symmetric spaces.
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*citation|first=K.|last= Nomizu|title= Invariant affine connections on homogeneous spaces|journal=Amer. J. Math.|volume= 76|year= 1954|pages=33-65
*citation|first=Atle|last=Selberg|authorlink=Atle Selberg|title=Harmonic analysis and discontinuous groups in weakly symmetric riemannian spaces, with applications to Dirchlet series|journal=J. Indian Math. Society|volume=20|year=1956|pages=47–87
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*citation|title=Harmonic Analysis on Commutative Spaces|first=Joseph A.|last= Wolf|publisher=American Mathematical Society|year= 2007
id=ISBN 0821842897