Bourbaki

Corollary

Tag 01CJ, permanent. Cite it as [Bourbaki, Tag 01CJ].

Lie Groups and Lie Algebras, chapter IX, § 4. Root system associated to a compact group

The set of g∈Gg\in G such that T∩gTg−1≠C(G)T\cap gTg^{-1}\not= C(G) is the union of a finite number of closed analytic submanifolds of G distinct from G.

Indeed, put Ag=T∩gTg−1A_g= T\cap gTg^{-1}; we have T⊂Z(Ag)T\subset Z(A_g) and gTg−1⊂Z(Ag)gTg^{-1}\subset Z(A_g). Hence, there exists x∈Z(Ag)x\in Z(A_g) such that xTx−1=gTg−1(xTx^{-1}=gTg^{-1}(§2, no. 2, Th. 2), which implies that g∈Z(Ag).NG(T)g\in Z(A_g).N_G(T). Denote by A\mathscr{A} the finite (Cor. 1) set of closed subgroups of G containing T and distinct from G, and put X = ⋃H∈AH.NG(T)\bigcup_{H\in\mathscr{A}}H.N_G(T); this is a finite union of closed submanifolds of G, distinct from G. If Ag≠C(G)A_g\not= C(G), then Z(Ag)∈AZ(A_g)\in \mathscr{A}, and gg belongs to X. Conversely, if g∈H.NGg\in H.N_G(T), with H∈AH\in \mathscr{A}, then AgA_g contains C(H), so Ag≠C(G)A_g\not= C(G) (Prop. 13).

Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.

Cited by

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Cites

Transcribed from the printed volume. Not translated.