Urysohn's lemma

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Notion

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Science and technology

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What Wikipedia says

In topology, Urysohn's lemma is a lemma that states that a topological space is normal if and only if any two disjoint closed subsets can be separated by a continuous function.

Excerpt from the Wikipedia article “Urysohn's lemma”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Roman Salinas, CC BY-SA 4.0, via Wikimedia Commons.