Condensation point

In mathematics, a condensation point p of a subset S of a topological space is any point p such that every neighborhood of p contains uncountably many points of S. Thus "condensation point" is synonymous with "[math]\displaystyle{ \aleph_1 }[/math]-accumulation point".[1]

## Examples

* If S = (0,1) is the open unit interval, a subset of the real numbers, then 0 is a condensation point of S.
* If S is an uncountable subset of a set X endowed with the indiscrete topology, then any point p of X is a condensation point of X as the only neighborhood of p is X itself.

## References

1. ↑ "Condensation point of a set - Encyclopedia of Mathematics". https://www.encyclopediaofmath.org/index.php/Condensation_point_of_a_set.

* Walter Rudin, Principles of Mathematical Analysis, 3rd Edition, Chapter 2, exercise 27
* John C. Oxtoby, Measure and Category, 2nd Edition (1980),
* Lynn Steen and J. Arthur Seebach, Jr., Counterexamples in Topology, 2nd Edition, pg. 4

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