Nowhere dense set: Difference between revisions
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In [[general topology]], a '''nowhere dense set''' in a topological space is a set whose [[closure ( | In [[general topology]], a '''nowhere dense set''' in a topological space is a set whose [[closure (topology)|closure]] has empty [[interior (topology)|interior]]. | ||
An [[infinite set|infinite]] [[Cartesian product]] of non-empty non-[[compact space]]s has the property that every compact subset is nowhere dense. | An [[infinite set|infinite]] [[Cartesian product]] of non-empty non-[[compact space]]s has the property that every compact subset is nowhere dense. | ||
Revision as of 14:30, 6 January 2009
In general topology, a nowhere dense set in a topological space is a set whose closure has empty interior.
An infinite Cartesian product of non-empty non-compact spaces has the property that every compact subset is nowhere dense.
A finite union of nowhere dense sets is again nowhere dense.
A first category space or meagre space is a countable union of nowhere dense sets: any other topological space is of second category. The Baire category theorem states that a non-empty complete metric space is of second category.
References
- J.L. Kelley (1955). General topology. van Nostrand, 145,201.