A space in which sequences have unique limits but compact sets need not be closed - MathOverflow

A space in which sequences have unique limits but compact sets need not be closed

2012-09-07T05:16:48Z

A topological space is KC if every compact subspace is closed. A topological space is US if every convergent sequences has exactly one limit. Does someone know an easy example of a US space which is not KC? Thanks.

Answer by Paul Fabel for A space in which sequences have unique limits but compact sets need not be closed

2012-09-07T08:25:56Z

To create a counterexample X, start with the closed interval [0,1] (with the usual topology) and attach a new point z whose neighborhoods are open dense subsets of [0,1].

Observe [0,1] is a compact nonclosed subspace of X and thus X is not a KC space. However no sequence in [0,1] converges to z and in particular all convergent sequences in X have unique limits.

The finite complement topology on an infinite set does not yield a counterexample since every infinite sequence converges to every point of the space.

In general no counterexample Y can be a sequential space since if Y is a sequential space then Y is a KC space iff Y is a US space. ( Recall Y is a sequential space if every nonclosed set B contains a convergent sequence whose limit lies outside B).

Answer by Paul Fabel for A space in which sequences have unique limits but compact sets need not be closed

2012-09-07T09:00:46Z

Start with the one point compactification of the minimal uncountable well ordered space and then split the maximum point into two points.

Answer by AliReza Olfati for A space in which sequences have unique limits but compact sets need not be closed

2012-09-07T11:28:10Z

I refer to COROLLARY 1 of This Article.

In COROLLARY 1 of it, $X^+$ denotes the one point compactification of the topological space $X$:

COROLLARY: Let $X$ be a Hausdorff space.Then:

(a) $X^+$ is always $US$.

(b) $X^+$ is $KC$ if and only if $X$ is a $\kappa$ space.

So it suffices to choose a Hausdorff space $X$, which is not $\kappa$ space. then $X^+$ is $US$ but is not $KC$.

PS: The topological space $X$ is called $\kappa$ space, if A subspace $A$ is closed in $X$ if and only if $A \cap K$ is closed in $K$, for all compact subset $K \subset X$.