Quotient-reflective and bireflective subcategories of the category of preordered sets

In previous papers, the notions of "closedness" and "strong closedness" in set-based topological categories were introduced. In this paper, we give the characterization of closed and strongly closed subobjects of an object in the category Prord of preordered sets and show that they form appropriate closure operators which enjoy the basic properties like idempotency (weak) hereditariness, and productivity. We investigate the relationships between these closure operators and the well-known ones, the up- and down-closures. As a consequence, we characterize each of T-0, T-1, and T-2 preordered sets and show that each of the full subcategories of each of T-0, T-1 T-2 preordered sets is quotient-reflective in Prord. Furthermore, we give the characterization of each of pre-Hausdorff preordered sets and zero-dimensional preordered sets, and show that there is an isomorphism of the full subcategory of zero-dimensional preordered sets and the full subcategory of pre-Hausdorff preordered sets. Finally, we show that both of these subcategories are bireflective in Prord. (C) 2011 Elsevier B.V. All rights reserved.