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Q40Section B (Part B)40 of 120

Topology — the tube lemma

Let XX and YY be topological spaces. Consider the following statement:

S: For every open subset U⊆X×YU\subseteq X\times Y and every x∈Xx\in X such that {x}×Y⊆U\{x\}\times Y\subseteq U, there is a neighbourhood WW of xx in XX such that W×Y⊆UW\times Y\subseteq U.

Which of the following statements is true?

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