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Sierpinski space∗ CWoo† 2013-03-21 13:03:59 Sierpinski space is the topological space X = {x, y} with the topology given by {X, {x}, ∅}. Sierpinski space is T0 but not T1 . It is T0 because {x} is the open set containing x but not y. It is not T1 because every open set U containing y (namely X) contains x (in other words, there is no open set containing y but not containing x). Remark. From the Sierpinski space, one can construct many non-T1 T0 spaces, simply by taking any set X with at least two elements, and take any nonempty proper subset U ⊂ X, and set the topology T on X by T = P (U ) ∪ {X}. ∗ hSierpinskiSpacei created: h2013-03-21i by: hCWooi version: h31222i Privacy setting: h1i hDefinitioni h54G20i † This text is available under the Creative Commons Attribution/Share-Alike License 3.0. You can reuse this document or portions thereof only if you do so under terms that are compatible with the CC-BY-SA license. 1