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Connected and Hausdorff topological space whose topology is stable under countable intersection,

Mathematics Asked on November 29, 2021

We know that the converging sequences of a discrete space are the stationary sequences.

I am looking for two examples for spaces (not empty or reduced to a singleton)

  1. connected and Hausdorff topological space where the converging sequences are the stationary sequences.
  2. connected and Hausdorff topological space whose topology is stable under countable intersection.

One Answer

There is an example in Arvind K. Misra, A topological view of P-spaces, General Topology and its Applications, Volume 2, Issue 4, December 1972, 349-362. It starts with the space $E_0$ that he constructs in Example $bf{3.1}$, a Hausdorff $P$-space (i.e., one in which $G_delta$-sets are open) with two points $a$ and $b$ that cannot be separated by a continuous function. In Example $bf{5.3}$ he recursively constructs from $E_0$ spaces $E_n$ for $ninomega$ in such a way that $E_n$ is embedded in $E_{n+1}$ and then defines $E_omega$ to be the direct limit of the sequence $langle E_n:ninomegarangle$. (The topology on $E_omega$ is the final topology determined by the embeddings.) $E_omega$ is a Hausdorff $P$-space on which every real-valued continuous function is constant, so it is connected.

In any $P$-space every convergent sequence is eventually constant.

Answered by Brian M. Scott on November 29, 2021

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