Complex Lie algebra
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In mathematics, a complex Lie algebra is a Lie algebra over the complex numbers.
Given a complex Lie algebra , its conjugate is a complex Lie algebra with the same underlying real vector space but with acting as instead.[1] As a real Lie algebra, a complex Lie algebra is trivially isomorphic to its conjugate. A complex Lie algebra is isomorphic to its conjugate if and only if it admits a real form (and is said to be defined over the real numbers).
Real form
Given a complex Lie algebra , a real Lie algebra is said to be a real form of if the complexification is isomorphic to .
A real form is abelian (resp. nilpotent, solvable, semisimple) if and only if is abelian (resp. nilpotent, solvable, semisimple).[2] On the other hand, a real form is simple if and only if either is simple or is of the form where are simple and are the conjugates of each other.[2]
The existence of a real form in a complex Lie algebra implies that is isomorphic to its conjugate;[1] indeed, if , then let denote the -linear isomorphism induced by complex conjugate and then
- ,
which is to say is in fact a -linear isomorphism.
Conversely,[clarification needed] suppose there is a -linear isomorphism ; without loss of generality, we can assume it is the identity function on the underlying real vector space. Then define , which is clearly a real Lie algebra. Each element in can be written uniquely as . Here, and similarly fixes . Hence, ; i.e., is a real form.
Complex Lie algebra of a complex Lie group
Let be a semisimple complex Lie algebra that is the Lie algebra of a complex Lie group . Let be a Cartan subalgebra of and the Lie subgroup corresponding to ; the conjugates of are called Cartan subgroups.
Suppose there is the decomposition given by a choice of positive roots. Then the exponential map defines an isomorphism from to a closed subgroup .[3] The Lie subgroup corresponding to the Borel subalgebra is closed and is the semidirect product of and ;[4] the conjugates of are called Borel subgroups.
Notes
References
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