Bourbaki

Corollary

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

Topologie algébrique, chapter III, § 2. Homotopie et chemins

Let X and B be topological spaces and let p:X→Bp: X\rightarrow B be a continuous, open, proper and separated mapping whose fibres are totally disconnected. Let I be an interval of R\mathbf{R}, let f:I→Bf: I\rightarrow B be a continuous mapping, let aa be a point of I and let xx be a point of X such that f(a)=p(x)f(a) =p(x). There exists a continuous mapping g:I→Xg: I\rightarrow X such that p∘g=fp\circ g=f and g(a)=xg(a) =x.

Posons X′=I×BXX'= I\times_BX and denote by p′:X′→p': X'\rightarrow I and f′:X′→f': X'\rightarrow X the canonical projections. The mapping p′p' is continuous, open, proper and separated (I, p. 17, prop. 8 and p. 27, prop. 4). Since the space I is separated, the space X′X' is separated (I, p. 26, remark 3). The fibres of p′p' are totally disconnected ( I, p. 10, corollary, a)). According to Theorem 3, there exists a continuous section s′s' of p′p' which takes in aa the value (a,x)(a, x). The mapping g=f′∘s′g=f'\circ s' of I into X is continuous, we have p∘g=p∘f′∘s′=f∘p′∘s′=fp\circ g=p\circ f'\circ s'=f\circ p'\circ s'=f and g(a)=xg(a) =x, whence the corollary.

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-5-6-mini, gpt-5-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.