This is a follow-up to my questions here and here. A subset of a topological space is called relatively compact if its closure is compact. Let's call a sequence $(U_n)$ of open sets a bounding sequence if the closure of $U_n$ is a subset of $U_{n+1}$ for all $n$ and every relatively compact set is a subset of some $U_n$. And let's call a set $S$ saturated if for every bounding sequence $(U_n)$, $S$ is a subset of some $U_n$. My question is, for what topological spaces is every saturated set relatively compact?
Is there some category of topological spaces which satisfies this property? And is there an example of a completely regular space which doesn’t satisfy this property?
My reason for asking this question, by the way, is that relatively compact sets form a bornology for $T_1$ spaces, and this property is the conditions for a bornology to be induced by a compatible uniformity, as I discuss here.
Best Answer
Clearly, the property is equivalent to that every closed saturated set is compact, so all compact spaces have the property. On the other hand, in a trivial case when the space with the property has no bounding sequences then it is saturated, so compact.
For a space $X$ put $X_c=\{x\in X:\overline{\{x\}}$ is compact$ \}$ and $$X^+_c=\bigcap\{U\subset X: X_c\subset\operatorname{int} U\}=\{x\in X: \overline{\{x\}}\cap X_c\ne\varnothing\}.$$ It is easy to check that each relatively compact subset of $X$ is contained in $X_c$. If $X$ is $T_1$ then $X_c=X$. Recently Paolo Lipparini suggested us to consider spaces $X$ with the property (P) that $X=X_c$ (for other reasons). He observed that:
Clearly, each bounding sequence of a space $X$ is a cover of $X^+_c$. Conversely, if $(U_n)$ is a cover of a set $X_c$ by open sets such that $\overline{U_n}\subset U_{n+1}$ for all $n$ then $(U_n)$ is a bounding sequence. Indeed, let $B$ be a relatively compact subset of $X$. Then $\overline{B}\subset X_c$ is compact. Thus $(U_n)$ is a non-decreasing open cover of a compact set $\overline{B}$, so there exists $n$ such that $B\subset \overline{B}\subset U_n$. Also remark that in this case a family $X^+_c\setminus \overline{U_n}$ is locally finite.
The above implies that each bounded subset of a space with property (P) is saturated. On the other hand, each saturated subset $B$ of $X$ is functionally bounded, that is if every continuous real-valued function defined on $X$ is bounded on $B$. Indeed, if $f:X\to\Bbb R$ is a continuous function unbounded on $B$, then $(f^{-1}(-n,n))$ is a bounding sequence witnessing that $B$ is not saturated. That is, a subset of a completely regular space is saturated iff it is functionally bounded. So a completely regular space has the property iff each its closed functionally bounded subset is compact. In particular, each pseudocompact space satisfying the property is compact. Recall that a subset $B$ of a normal space $X$ is functionally bounded iff $B$ is pseudocompact. Indeed, if $B$ is not pseudocompact then there exists a continuous real-valued unbounded function $f$ on $B$. Since $B$ is a closed subset of a normal space, by Tietze-Urysohn’s Theorem $f$ can be extended to a continuous real-valued (unbounded) function on the whole space, which violates functional boundedness of $B$.
Thus space $X$ has the property under the known condition assuring a pseudocompact space is compact.
$X$ is paracompact. Indeed, by [Eng, Theorem 5.1.5] $X$ is normal. Let $B$ be any closed saturated subset of $X$. The set $B$ is paracompact as a closed subset of a paracompact space. Since $B$ is feebly compact, it is compact.
$X$ is a normal space with $G_\delta$-diagonal. Indeed, let $B$ be any closed saturated subset of $X$. Since $B$ is feebly compact and normal, by [Eng, Theorem 4.3.28] $B$ is countably compact. By Chaber’s Theorem [Gru, Theorem 2.14], $B$ is compact. See, for instance this my answer for the formulations and proofs of the mentioned theorems.
References
[Eng] Ryszard Engelking, General Topology, 2nd ed., Heldermann, Berlin, 1989.
[Gru] Gary Gruenhage Generalized Metric Spaces, in: K.Kunen, J.E.Vaughan (eds.) Handbook of Set-theoretic Topology, Elsevier Science Publishers B.V., 1984.
[TS] Iván Sánchez, Mikhail Tkachenko, Products of bounded subsets of paratopological groups, Topology Appl., 190 (2015), 42-58.