For any completely regular space , by we mean the space of all real-valued continuous functions on endowed with the pointwise convergent topology. It is known that is hereditarily separable if and only if is hereditarily Lindelof for all positive integer if and only if is hereditarily Lindelof where is the first infinite ordinal (a result that follows from a theorem of Zenor in ). This result points to a duality between hereditary separability of the function space and the hereditary Lindelof property of the domain space and is restated below.
Let be a completely regular space. Then the following conditions are equivalent:
- is hereditarily separable.
- is hereditarily Lindelof for all positive integer .
- is hereditarily Lindelof.
As an introduction to this theorem, we present the proof to one direction of this theorem for .
Let be any completely regular space. We have the following obervation:
If is hereditarily separable, then is hereditarily Lindelof.
Suppose is not hereditarily Lindelof. We aim to show that is not hereditarily separable by producing a non-separable subspace of .
Let be a subspace that is not Lindelof. Let be a collection of open subsets of such that covers and no countable subcollection of covers .
For each , choose such that . By the completely regularity of , choose a continuous such that maps to and . Let . It can be shown that is a non-separable subspace of . That is, no countable subset of can be dense in .
For any completely regular space , it is also known (see ) that is separable if and only if has a weaker separable metrizable topology (i.e. has a weaker topology such that with this weaker topology is a separable metrizable space). The result in  combined with the observation presented here provides a way to obtain sepearable that is not hereditarily separable. Look for any that is not hereditarily Lindelof but has a weaker separable metrizable topology. One such example is the Michael Line.
The observation we make here is a rather weak result. The double arrow space is hereditarily Lindelof. Yet is not even separable since is compact space that is not metrizable. Note that is not hereditarily Lindelof since it contains a copy of the Sorgenfrey plane (see the previous post on double arrow space).
- Engelking, R., General Topology, Revised and Completed edition, 1989, Heldermann Verlag, Berlin.
- Noble, N., The density character of function spaces, Proc. Amer. Math. Soc. 42:1 (1974) 228-233.
- Willard, S., General Topology, 1970, Addison-Wesley Publishing Company.
- Zenor, P., Hereditarily m-separability and the hereditarily m-lindelof property in product spaces and function spaces, Fund. Math. 106 (1980), 175-180