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# dense in-itself

A subset $A$ of a topological space is said to be *dense-in-itself* if $A$ contains no isolated points.

Note that if the subset $A$ is also a closed set, then $A$ will be a perfect set. Conversely, every perfect set is dense-in-itself.

A simple example of a set which is dense-in-itself but not closed (and hence not a perfect set) is the subset of irrational numbers. This set is dense-in-itself because every neighborhood of an irrational number $x$ contains at least one other irrational number $y\neq x$. On the other hand, this set of irrationals is not closed because every rational number lies in its closure.

For similar reasons, the set of rational numbers is also dense-in-itself but not closed.

Related:

ScatteredSpace

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Definition

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## Mathematics Subject Classification

54A99*no label found*

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## Recent Activity

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new correction: Error in proof of Proposition 2 by alex2907

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new correction: Error in proof of Proposition 2 by alex2907

Jun 24

new question: A good question by Ron Castillo

Jun 23

new question: A trascendental number. by Ron Castillo

Jun 19

new question: Banach lattice valued Bochner integrals by math ias

Jun 13

new question: young tableau and young projectors by zmth

Jun 11

new question: binomial coefficients: is this a known relation? by pfb

Jun 6

new question: difference of a function and a finite sum by pfb