D-completions and the d-topology

In this article we give a general categorical construction via reflection functors for various completions of T 0 -spaces subordinate to sobrification, with a particular emphasis on what we call the D -completion, a type of directed completion introduced by Wyler [O. Wyler, Dedekind complete posets and Scott topologies, in: B. Banaschewski, R.-E. Hoffmann (Eds.), Continuous Lattices Proceedings, Bremen 1979, in: Lecture Notes in Mathematics, vol. 871, Springer Verlag, 1981, pp. 384–389]. A key result is that all completions of a certain type are universal, hence unique (up to homeomorphism). We give a direct definition of the D -completion and develop its theory by introducing a variant of the Scott topology, which we call the d -topology. For partially ordered sets the D -completion turns out to be a natural dcpo-completion that generalizes the rounded ideal completion. In the latter part of the paper we consider settings in which the D -completion agrees with the sobrification respectively the closed ideal completion.