Recall that a Peano space is a connected, locally connected, compact metrizable space. The name comes from the fact that these are exactly the spaces that can be filled by curves; cf. Peano’s discovery that there exist curves that fill the space :
(Hahn-Mazurkiewicz) For a nonempty Hausdorff space , there exists a continuous surjection from the unit interval if and only if is a Peano space.
Perhaps it should be pointed out that these are exactly the (Hausdorff) quotients of : that if is a continuous surjection, then is in fact a quotient map (simply because is a closed surjective map).
We get a wider class of spaces by considering (Hausdorff) quotients of instead.
Already I’m getting tired of putting “Hausdorff” in parentheses, so let’s agree in this article that we henceforth consider only nonempty Hausdorff spaces.
Note that itself is a quotient of , via the composite
where the first map is a covering projection and the second map is a branched projection map. It follows that every Peano space is a quotient of .
I will aim to prove the following result:
A space is a quotient of if and only if it is a filtered colimit of Peano spaces.
It should be observed that quotients of locally connected spaces are locally connected.
There is a continuous surjection if and only if is path-connected and covered by countably many Peano subspaces. (N.B.: the topology on need not be the coherent topology induced from the union.)
In the forward direction, given a continuous surjection , the space is covered by Peano spaces where ranges over . In the backward direction: given Peano subspaces that cover , we can construct surjective maps , and then extend the union of these maps to an (evidently surjective) function by letting the restriction of to the interval be a chosen path from to .
Every quotient of can be expressed as a -colimit of a countable chain of Peano spaces.
This is essentially obvious: suppose given a quotient map , and let as a subspace of . Then the restriction, call it , is a quotient map, so is a Peano space. By compatibility of the restriction maps, the identity function
is a continuous function if we regard the domain as endowed with the coherent or colimit topology (induced by the countable chain of inclusions ). This identity function is also open: since is a quotient map, the continuity of the inverse is equivalent to the continuity of
but since the topology of is the coherent topology induced from the chain of inclusions , this boils back down to the continuity of the and the compatibility between them.
A colimit of a countable chain of Peano spaces is a quotient of .
Observe that each is path-connected. We construct by induction a sequence of quotient maps such that is the restriction of along the inclusion . Indeed, there exists a quotient map , and by path-connectedness there exists an extension of the union of the maps
to a map , evidently a quotient map. The maps paste together to give a surjective map . To see is a quotient map, suppose is open for some given . Let be the cocone inclusion; then
is open in , hence is open in since is a quotient map, and thus is open in since has the coherent topology induced from the .
An LP-space is a colimit in of a countable chain of Peano spaces.
The category of LP-spaces has colimits of countable chains.