Proposition 3.39 of Hall's Lie Groups, Lie Algebras And Representations:
"Let g be a real Lie algebra, gC its complexification, and h an
arbitrary complex Lie algebra. Then every real Lie algebra homomorphism of g into h extends uniquely to a complex Lie algebra homomorphism of gC into h."
In particular this means that any real representation of g defines a complex representation of gC.
Question: Does the converse hold? Does any complex representation of gC define a real representation of g? Are there any conditions for when this may or may not hold?
Answer
Of course it does. If the Lie algebra gC acts on a complex vector space V, simply consider the restriction of this action to g. To be more precise, if X∈g and if v∈V, then define X.v as (X⊗1).v. I am assuming here that Hall defined gC as g⨂RC. If he used another definition, pleas say which one does he use.
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