Condensation point

In mathematics, a condensation point p of a subset S of a topological space, is any point p, such that every open neighborhood of p contains uncountably many points of S. Thus, "condensation point" is synonymous with "-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 open neighborhood of p is X itself.

References
  1. 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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