Proposition 2

Tag 0DO7, permanent. Cite it as [Bourbaki, Tag 0DO7].

Algebra, chapter IX, § 10. Angles

The kernel of the canonical homomorphism of $O^+$ into $S^+/H$ is $\{1, -1\}$ .

In fact 1 and $-1$ are the only elements of $H \cap O^+$ .

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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