dense set
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subset whose closure is the whole space
~11 min read
Encyclopedic overview
In topology and related areas of mathematics, a subset A of a topological space X is said to be dense in X if every point of X either belongs to A or else is arbitrarily "close" to members of A — for instance, the rational numbers are a dense subset of the real numbers because every real number either is a rational number or has a rational number arbitrarily close to it (see Diophantine approximation). Formally,
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Excerpted from Wikipedia’s “dense set” article, available under the CC BY-SA 4.0 licence.