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 one 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. The rank is always at least one, with equality if the sectional curvature is positive or negative. If the curvature is positive, the space is of compact type, and if negative, it is of noncompact type. 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, 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.
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