Bourbaki

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 i∈Ii\in I, the group π1(Ai,a(i))\pi_1(A_i, a(i)) is reduced to the identity element and that, for every triple (i1,i2,i3)(i_1, i_2, i_3) of pairwise distinct elements of I, the set Ai1∩Ai2∩Ai3A_{i_1}\cap A_{i_2}\cap A_{i_3} is empty. The homomorphism M′′:\mathsf{M}'': F(J-Fl(T)) →π1(Y,a(i0))\rightarrow \pi_1(Y, a(i_0)) induced by M\mathsf{M} by restriction is then an isomorphism.

Let π′:F(J)→\pi ': F(J)\rightarrow F(J - Fl(T)) be the homomorphism which maps [j][j] to [j][j] if j∈j\in J-Fl(T) and which maps [j][j] to the identity element if j∈j\in Fl(T); it is surjective and one has M′′∘π′=M′\mathsf{M}''\circ \pi '=\mathsf{M}', where M′\mathsf{M}' is the surjective homomorphism defined in corollary 1. It follows that M′′\mathsf{M}'' is surjective and that its kernel is the smallest distinguished subgroup of F(J-Fl(T)) which contains the images under π′\pi ' of the elements described in loc. cit. But, by construction, π′(j)=e\pi '(j) =e if j∈j\in Fl(T) and the set K is empty, by hypothesis. The homomorphism M′′\mathsf{M}'' 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.

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Cites

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Machine translation of the English, by gpt-5-6-mini, gpt-5-6, gpt-5.4, against glossary version 34. Not checked by a person. That model is a cut down version of the one the rest of this language was written by.