Corollary 1
Tag 06P0, permanent. Cite it as [Bourbaki, Tag 06P0].
Algebra, chapter V, § 11. Abelian extensions
— If G is a commutative group annihilated by n, and , then .
For it suffices to note that if H is a cyclic subgroup of G distinct from , then , and to apply Prop. 7.
Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.