Bourbaki

Exercise 14, § 2

Tag 03ZO, permanent. Cite it as [Bourbaki, Tag 03ZO].

Algebraic Topology, III, p. 333

Let XX be a set and let c\mathfrak{c} be an infinite cardinal such that c<\mathfrak{c}< Card(X).

a) Let Uc\mathscr{U}_{\mathfrak{c}} be the set of parts V of X such that Card(X-V) ⩽c\leqslant \mathfrak{c} or V=∅V =\emptyset. Exercise prove that Uc\mathscr{U}_{\mathfrak{c}} is a topology on X.

b) Now prove that the set X, endowed with this topology, is connected and locally connected, but that its arcwise connected components are reduced to one element.

c) Now prove that the cone C(X) is arcwise connected, locally connected, but is not locally arcwise connected. This demonstrates that in corollary 2 of III, p. 267, one cannot omit the assumption that the space is locally compact and metrizable.

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