[Math] Are any of these notions of “k-space” equivalent if $X$ is not assumed weakly Hausdorff

general-topology

Is there a modern generally accepted answer regarding the notions of k-space or compactly generated space?

For example there are currently at least 3 formally distinct notions of k-space in wide circulation:

  1. In Kelley's General Topology, $X$ is a k-space if for $S \subseteq X$ not closed in $X$ there is a closed compact subspace $C \subseteq X$ such that $C \cap S$ is not closed in $X$.

    (This notion of k-space also appears in A. Wilansky's _Between T1 nd T2 (Amer. Math. Monthly, vol.74, no.3, pp.261-266).)

  2. According to nLab, $X$ is a k-space if whenever $S \subseteq X$ is not closed in $X$, there exists a compact Hausdorff space $K$ and a map $f:K \to X$ such that the preimage of $S$ is not compact.

    This is equivalent to $X$ being compactly generated (CG) in Neil Strickland's note The category of CGWH spaces.

  3. Wikipedia declares that $X$ is a k-space (or a compactly generated space) provided that whenever $S \subseteq X$ is not closed in $X$, then there exists a compact subspace $C$ of $X$, such that the intersection of $C$ and $S$ is not compact.

Are any of definitions 1,2,3 equivalent if $X$ is not weakly Hausdorff?

Best Answer

I think the Wikipedia definition is not the best, since it does not deal nicely with the non-Hausdorff case, and a quotient of a Hausdorff space need not be Hausdorff. This is kind of related to the question of whether locally compact means each point has a compact neighbourhood, or has a base of compact neighbourhoods. The latter concept is more in tune with the notion of a local property.

A general discussion of "Monoidal closed categories of spaces" is in a paper by Booth and Tillotson (Pacific J Math, vol 88) available here.

Section 5.9 of my book Topology and groupoids (as in the 1988 differently titled edition) has the following result:

5.9.1 Let $X$ be a space. Then the following are equivalent:

(a) $X$ is a $k$-space;

(b) there is a set $\mathcal C_{X}$ of maps $t : C_{t} \to X$ for compact Hausdorff spaces $C_t$ such that a set $A$ is closed in $X$ if and only if $t^{-1}(A)$ is closed in $C_{t}$ for all $t \in \mathcal C_{X}$;

(c) $X$ is an identification space of a space which is a sum of compact Hausdorff spaces.

So the n-lab definition agrees with this. Note that this Section also considers the convenient category of $k$-continuous functions, using the test-open topology on spaces of k-continuous maps.

Actually the idea of fibred exponential laws (i.e. some notion of locally cartesian closed) comes from a paper of Thom on "Homologie des espaces functionels" but the details were sketchy, and were developed by Peter Booth.

Related Question