# Difference between revisions of "Lie group, semi-simple"

A connected Lie group that does not contain non-trivial connected solvable (or, equivalently, connected Abelian) normal subgroups. A connected Lie group is semi-simple if and only if its Lie algebra is semi-simple (cf. Lie algebra, semi-simple). A connected Lie group is said to be simple if its Lie algebra is simple, that is, if does not contain non-trivial connected normal subgroups other than . A connected Lie group is semi-simple if and only if it splits into a locally direct product of simple non-Abelian normal subgroups.

The classification of semi-simple Lie groups reduces to the local classification, that is, to the classification of semi-simple Lie algebras (cf. Lie algebra, semi-simple), and also to the global classification of the Lie groups that correspond to a given semi-simple Lie algebra .

In the case of Lie groups over the field of complex numbers the main result of the local classification is that every simply-connected simple non-Abelian complex Lie group is isomorphic to one of the groups , , , (the universal covering of the group ), , (see Classical group), or one of the exceptional complex Lie groups (see Lie algebra, exceptional). The global classification of the Lie groups corresponding to a semi-simple Lie algebra over goes as follows. Let be a Cartan subalgebra of and let be the root system of with respect to . To every semi-simple Lie group with Lie algebra corresponds a lattice that is the kernel of the exponential mapping . In particular, if is simply connected, then coincides with the lattice generated by the elements , (see Lie algebra, semi-simple), and if is a group without centre (an adjoint group), then is the lattice

In the general case . For any additive subgroup satisfying the condition there is a unique (up to isomorphism) connected Lie group with Lie algebra such that . The centre of is isomorphic to , and for the fundamental group one has:

The quotient group (the centre of the simply-connected Lie group with Lie algebra ) is finite and for the different types of simple Lie algebras it has the following form:'

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 , ,

The order of the group is the same as the number of vertices with coefficient 1 in the extended Dynkin diagram of ; discarding one of the vertices gives the Dynkin diagram. A similar classification holds for compact real semi-simple Lie groups, each of which is imbedded in a unique complex semi-simple Lie group as a maximal compact subgroup (see Lie group, compact).

The global classification of non-compact real semi-simple Lie groups can be carried out in a similar but more complicated way. In particular, the centre of the simply-connected Lie group corresponding to a semi-simple Lie algebra over can be calculated as follows. Let be the Cartan decomposition, where is a maximal compact subalgebra of and is its orthogonal complement with respect to the Killing form, let be the corresponding involutive automorphism, extended to , the Cartan subalgebra of containing a Cartan subalgebra , an automorphism of that coincides with on the roots with respect to and extended to the root vectors in an appropriate way, and the Cartan decomposition of the real form corresponding to . Then (see [3], where this group is calculated for all types of simple algebras over ).

Every complex semi-simple Lie group has the unique structure of an affine algebraic group compatible with the analytic structure specified on it, and any analytic homomorphism of to an algebraic group is rational. The corresponding algebra of regular functions on coincides with the algebra of holomorphic representation functions. On the other hand, a non-compact real semi-simple Lie group does not always admit a faithful linear representation — the simplest example is the simply-connected Lie group corresponding to the Lie algebra . If is a semi-simple Lie algebra over , then in the centre of the simply-connected group corresponding to there is a smallest subgroup , called the linearizer, such that is isomorphic to a linear semi-simple Lie group. If is the compact real form of , then

(see [3], where this group is calculated for all types of simple Lie algebras ).

#### References

 [1] J.F. Adams, "Lectures on Lie groups" , Benjamin (1969) MR0252560 Zbl 0206.31604 [2] J.-P. Serre, "Lie algebras and Lie groups" , Benjamin (1965) (Translated from French) MR0218496 Zbl 0132.27803 [3] A.I. Sirota, A.S. Solodovnikov, "Noncompact semisimple Lie groups" Russian Math. Surveys , 18 : 3 (1963) pp. 85–140 Uspekhi Mat. Nauk , 18 : 3 (1963) pp. 87–144 MR155929 Zbl 0132.02101