Non-Hausdorff Topology and Domain Theory

Recently, Matthew de Brecht sent me a proof of a neat and rather surprising result: the product and the Scott topologies coincide for products of first-countable, not necessarily continuous, posets. This rests on a clever argument, inspired by techniques invented by Matthias Schröder, and a simple observation: if you take all the elements of a convergent sequence, plus one (any) of its limits, what you get is a compact set. The latter fails if you take a net instead of a sequence. Read the full post.

Johnstone space J (1981) is the most famous example of a non-sober dcpo. In 1982, Isbell came up with a non-sober complete lattice. His construction is so complex that most authors use it as a black box. I would like to explain how Isbell’s non-sober complete lattice is constructed. As you can expect, this is a pretty clever construction, but I claim this is understandable. I will conclude with remarkable results of Xu, Xi, and Zhao (2019), who proved that there even exists a non-sober frame. Read the full post.

Last time, I had announced that we would do Exercise V-5.25 of the red book, constructing a core-compact, yet not locally compact, space.  And this is exactly what we shall do: read the full post.

This month, we will start to do Exercise V-5.25 of the red book (Continuous Lattices and Domains), which gives an example of a core-compact, not locally compact space.  That is pretty hard to obtain, really.  This month, we will do the first step of that exercise by constructing Bernstein sets, which are very, very pathological subsets of R.  Read the full post.

Let me first wish you a Merry Christmas, and since I will not post again next week, a Happy New Year 2019 as well.  I have no specific present this year, sorry… This month’s post is about a few thoughts I have had about the role of countability is a few situations other than the expected ones.  This will culminate in a recent result of de Brecht and Kawai—the Scott and the upper Vietoris topologies coincide on the Smyth powerdomain of a well-filtered second-countable space— and its clever proof: see the full post.

Last time, we embarked on proving that the projective limit of a projective system of compact sober (resp., and non-empty) spaces is compact and sober (resp., and non-empty), a theorem that Fujiwara and Kato call Steenrod’s Theorem.  However, instead, we merely proved that a projective limit of a projective system of non-empty compact sober spaces is non-empty.  Do not despair: this is the essential argument in the proof of Steenrod’s Theorem, which we complete this month.  Read the full post.