Corollary 2
Tag 0236, permanent. Cite it as [Bourbaki, Tag 0236].
Topologie algébrique, chapter IV, § 5. Théorème de van Kampen
Under the assumptions of proposition 1, suppose moreover that for every , the group is reduced to the identity element and that, for every triple of pairwise distinct elements of I, the set is empty. The homomorphism F(J-Fl(T)) induced by by restriction is then an isomorphism.
Let F(J - Fl(T)) be the homomorphism which maps to if J-Fl(T) and which maps to the identity element if Fl(T); it is surjective and one has , where is the surjective homomorphism defined in corollary 1. It follows that is surjective and that its kernel is the smallest distinguished subgroup of F(J-Fl(T)) which contains the images under of the elements described in loc. cit. But, by construction, if Fl(T) and the set K is empty, by hypothesis. The homomorphism is therefore an isomorphism.
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
Nothing.
Cites
Nothing.