# Are any of these notions of “k-space” equivalent if $X$ is not assumed weakly Hausdorff?

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?

• I don't think it makes sense to define compactly generated to mean anything other than the nLab's definition. – Qiaochu Yuan Oct 3 '12 at 21:50
• Thanks Qiaochu. I assume by you mean is likely to prove most useful''. It is a little unclear if nlab is trying to define k-space or compactly generated or both. The wikipedia definition is suspect. – Paul Fabel Oct 3 '12 at 22:52
• There’s nothing suspect about the Wikipedia definition: it’s the one found in Willard, for example, and it’s perhaps the most obvious interpretation of the term compactly generated. – Brian M. Scott Oct 3 '12 at 23:57
• By suspect' I mean having good reason for being questioned or challenged' as indicated in the original post. In particular two modern treatises (nlab and Neil Strickland's notes) define compactly generated in a stronger manner than wikipedia. Are they equivalent for general spaces? If not, I would suggest the wikipedia entry is indeed suspect in the sense of being contrary to modern usage. – Paul Fabel Oct 4 '12 at 3:52
• Moreover the same wikipedia ALSO indicates that compactly generated is equivalent to being a k-space, and wikipedia uses a different definition than the one established in `Kelley'. So that is a 2nd and independent reason for casting suspicion on the wikipedia entry. – Paul Fabel Oct 4 '12 at 3:54

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.