Limit point, Accumulation point, and Condensation point of a set

The three notions mentioned above should be clearly distinguished.

If $A$ is a subset of a topological space $X$ and $x$ is a point of $X$, then $x$ is an accumulation point of $A$ if and only if every neighbourhood of $x$ intersects $A\backslash \{x\}$.

It is a condensation point of $A$ if and only if every neighbourhood of it contains uncountably many points of $A$.

The term limit point is slightly ambiguous. One might call $x$ a limit point of $A$ if every neighbourhood of $x$ contains infinitely many points of $A$, but this is not standard.

Wikipedia: Let ${S}$ be a subset of a topological space $X$. A point $x$ in $X$ is a limit point of $S$ if every neighbourhood of $x$ contains at least one point of $S$ different from $x$ itself.

posted @ 2017-10-28 10:11  Jun-Hui Li  阅读(583)  评论(0编辑  收藏  举报