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  • 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 $Aackslash {x}$.

    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.

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  • 原文地址:https://www.cnblogs.com/aujun/p/5026285.html
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