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「連結だがある1点を除くと完全不連結になるようなEuclid平面の部分集合」ウケる(Cantor's leaky tent, Knaster-Kuratowski fan)

証明はもちろんCounterexamples in Topologyに載っている(連結性の証明はけっこう長い)

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Mathtodon

A Mastodon instance named Mathtodon, where you can post toots with beautiful mathematical formulae in TeX/LaTeX style.