Bourbaki

Corollary 2

Tag 01Q4, permanent. Cite it as [Bourbaki, Tag 01Q4].

Topologie algébrique, chapter I, § 4. Revêtements

Let B be a topological space and A a subspace of B. Suppose that the spaces A and B are locally connected and that the pair (B,A)(B,A) has property (PCV) of I, p. 37. Let E and E′E' be coverings of B, and let us denote by pp and p′p' their projections. Let

g:p−1(A)→(−p′1)(A)g:\overset{-1}{p}(A)\rightarrow (^-{p'}^1)(A) be an A-morphism ( resp. an A-isomorphism). Then there exists a neighbourhood U of A in B and a U-morphism ( resp. a

U-isomorphism) f:p−1(U)→(p−1′)(U)f:\overset{-1}{p}(U)\rightarrow (\overset{-1}{p}')(U) such that fA=gf_A=g.

Let us retain the notation of corollary 1. By that same corollary, an A-morphism g:EA→EA′g: E_A\rightarrow E'_A is identified with a section s0s_0 above A of the étale space EME_{\mathscr{M}} associated with M\mathscr{M}. By the hypothesis made on the pair (B,A)(B,A) and lemma 3 of I, p. 39, there exists an open neighbourhood U of A in B and a continuous section ss of EME_{\mathscr{M}} above U extending s0s_0. This section ss is identified with a U-morphism f:EU→EU′f: E_U\rightarrow E'_U extending ff.

The case where gg is an A-isomorphism is treated analogously by considering, in place of the sheaf M\mathscr{M}, the sheaf I\mathscr{I}.

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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Machine translation of the English, by gpt-5.4, against glossary version 34. Not checked by a person.