Door space

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In mathematics, in the field of topology, a topological space is said to be a door space if every subset is either open or closed (or both).[1] The term comes from the introductory topology mnemonic that "a subset is not like a door: it can be open, closed, both, or neither".

Here are some easy facts about door spaces:

To prove the first assertion, let X be a Hausdorff door space, and let x ≠ y be distinct points. Since X is Hausdorff there are open neighborhoods U and V of x and y respectively such that U∩V=∅. Suppose y is an accumulation point. Then U\{x}∪{y} is closed, since if it were open, then we could say that {y}=(U\{x}∪{y})∩V is open, contradicting that y is an accumulation point. So we conclude that as U\{x}∪{y} is closed, X\(U\{x}∪{y}) is open and hence {x}=U∩[X\(U\{x}∪{y})] is open, implying that x is not an accumulation point.

Notes[edit]

  1. ^ Kelley, ch.2, Exercise C, p. 76.

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