Lingering sequence

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Given a metric space [ilmath](X,d)[/ilmath] a lingering sequence or sometimes hovering sequence is a sequence [ilmath](x_n)_{n=1}^\infty\subseteq X[/ilmath] that satisfies the following property[1]:

  • [ilmath]\exists x\in X\forall\epsilon>0[\vert B_\epsilon(x)\cap (x_n)_{n=1}^\infty\vert=\aleph_0][/ilmath]

Or in words:

  • This


Let [ilmath](X,d)[/ilmath] be a metric space, then[2]:

  • [math]\forall(x_n)_{n=1}^\infty\subseteq X\left[\left(\exists x\in X\ \forall\epsilon>0[\vert B_\epsilon(x)\cap(x_n)_{n=1}^\infty\vert=\aleph_0]\right)\implies\left(\exists(k_n)_{n=1}^\infty\subseteq\mathbb{N}\left[(\forall n\in\mathbb{N}[k_n<k_{n+1}])\implies\left(\exists x'\in X\left[\lim_{n\rightarrow\infty}(x_{k_n})=x'\right]\right)\right]\right)\right][/math]

This is just a verbose way of expressing the statement that:

  • Given a sequence [ilmath](x_n)_{n=1}^\infty\subseteq X[/ilmath] if it is a lingering sequence then it has a subsequence that converges

In a metric space [ilmath](X,d)[/ilmath] that is compact every sequence is a lingering sequence, that is to say[2]:

  • [math]\forall(x_n)_{n=1}^\infty\subseteq X\ :\ \exists x\in X\ \forall\epsilon>0[\vert B_\epsilon(x)\cap(x_n)_{n=1}^\infty\vert=\aleph_0][/math]



  1. Alec's own work
  2. 2.0 2.1 Introduction to Topology - Theodore W. Gamelin & Robert Everist Greene