Bourbaki

Corollary 2

Tag 0CCL, permanent. Cite it as [Bourbaki, Tag 0CCL].

General Topology, chapter X, § 2. Equicontinuous sets

Let XX be a topological (resp. uniform) space, let YY be a complete Hausdorff uniform space and let HH be an equicontinuous (resp. uniformly equicontinuous) subset of C(X;Y)C(X; Y). Suppose that H(x)H(x) is relatively compact in YY for all x∈Dx \in D, where DD is a dense subset of XX. Then HH is relatively compact in C(X;Y)C(X; Y) with respect to the topology of compact (resp. precompact) convergence.

It is enough to show that H(x)H(x) is relatively compact for all x∈Xx \in X, for we can then apply Corollary 1. Since YY is complete it is enough to show that H(x)H(x) is precompact for all x∈Xx \in X. Now if VV is any symmetric entourage of YY, there is a neighbourhood UU of xx such that (u(x),u(x′))∈V(u(x), u(x')) \in V for all x′∈Ux' \in U and all u∈Hu \in H. By hypothesis there exists x′∈U∩Dx' \in U \cap D, and since H(x′)H(x') is relatively compact in YY, there exists a finite number of points yk∈Yy_k \in Y such that H(x′)H(x') is contained in the union of the sets V(yk)V(y_k); hence H(x)H(x) is contained in the union of the sets V(yk)V(y_k), and the proof is complete.

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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Cites

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