Formal Topological Characterizations of Various Continuous Domains$
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Computers and Mathematics with Applications 56 (2008) 444–452 www.elsevier.com/locate/camwa Formal topological characterizations of various continuous domains$ Luoshan Xua,∗, Xuxin Maob a Department of Mathematics, Yangzhou University, Yangzhou 225002, PR China b College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 15 August 2005; received in revised form 2 April 2007; accepted 4 December 2007 Abstract In this paper, some new concepts such as (super-compact) quasi-bases and the consistently (locally) coherent property of super- compact quasi-bases, are introduced. With these concepts, various continuous domains including bc-domains and (s)L-domains, are successfully characterized in formal topological ways. Furthermore, to deal with algebraic domains, the concept of quasi- formal points, a generalization of formal points, is introduced. Formal topological characterizations of various algebraic domains via quasi-formal points are obtained. c 2008 Elsevier Ltd. All rights reserved. Keywords: Continuous domain; Locally super-coherent topology; Quasi-base; (Quasi-)formal point; BC-domain; L-domain 1. Introduction In 1972, Dana Scott introduced a class of lattices called continuous lattices in order to provide models for the type free calculus in logic (see [1]). Later, a more general notion of continuous domains, which are mathematical structures used in semantics as carriers of meaning, was introduced in [2]. Now domain theory has received more and more attention (see [3–8]) of both mathematicians and computer scientists. And the study of domain theory integrates the studies of order structures, topological structures as well as algebraic structures. Meanwhile, domain theory is also closely related to theoretical computer science and yields many applications in various areas. It should be noted that a distinctive feature of the theory of continuous domains is that many of the considerations are closely interlinked with topological ideas (see [3,4,8,9]). The Scott topology, as an order-theoretical topology, is of fundamental importance in domain theory. It lies at the heart of the structure of continuous domains. In [2], Lawson proved that a dcpo is continuous if and only if its Scott topology is completely distributive. However, the definitions or characterizations of the Scott topology always assume the underlying set to be equipped with a partial order. Domain theory can be seen as a branch of formal topology which is the topology as developed in (Martin Lof’s)¨ type theory. Recently, Sambin in [10], introduced the concept of super-coherent topology. A purely topological characterization of the Scott topologies over algebraic dcpos, independently of any orders, was presented. $ Supported by NSFC (10371106, 60774073), the Tianyuan Fund of Mathematics of NSFC (10726028) and the Fund (S0667-082) from Nanjing University of Aeronautics and Astronautics. ∗ Corresponding author. E-mail addresses: [email protected], [email protected] (L. Xu). 0898-1221/$ - see front matter c 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.camwa.2007.12.010 L. Xu, X. Mao / Computers and Mathematics with Applications 56 (2008) 444–452 445 Since continuous domains are more general and complicated than algebraic ones, it is tempting to find similar topological characterizations of various kinds of continuous domains. For this purpose, in this paper, a series of new concepts such as the locally super-coherent topology, (super-compact) quasi-bases and the consistently (resp., locally) coherent property of super-compact quasi-bases, are introduced. With these concepts, various continuous domains including bc-domains, sL-domains and L-domains, are successfully characterized in formal topological ways. Furthermore, to deal with algebraic domains, the concept of quasi-formal points, a generalization of formal points in some sense, is introduced. And formal topological characterizations of various algebraic domains via quasi-formal points are also thus obtained. 2. Preliminaries Recall that in a poset P, a subset D ⊆ P is directed if each finite subset of D has an upper bound in D. Any directed lower set of P is called an ideal. The set of all ideals of P ordered by set inclusion is called the ideal completion of P, denoted by I dl(P). A poset is called a directed complete poset (briefly dcpo) if each directed subset has a supremum. A poset is said to be bounded complete if every subset with an upper bound has a supremum. This implies that a bounded complete poset has a bottom. A complete lattice is a poset in which every subset has a supremum. Let P be a dcpo and x, y ∈ P. We say that x approximates y, written x y, if whenever D is directed with sup D ≥ y, then x ≤ d for some d ∈ D. An element k ∈ P is compact if k k. The set of all compact elements of P is denoted by K (P). If for every element x ∈ P, the set ↓x := {a ∈ P : a x} is directed and sup ↓x = x, then P is called a continuous domain. A dcpo is called an algebraic domain if every element is the directed supremum of compact elements that approximate it. A bounded complete continuous (algebraic) domain is called a bc-domain (Scott domain). A(n) continuous (algebraic) lattice means a(n) continuous (algebraic) domain which is a complete lattice. An (algebraic) sL-domain (see [6]) is a(n) continuous (algebraic) domain in which every principal ideal is a join-semilattice. An (algebraic) L-domain is a(n) continuous (algebraic) domain in which every principal ideal is a complete lattice. An upper set U of a dcpo P is said to be Scott open if for all directed sets D ⊆ P, sup D ∈ U implies U ∩ D 6= ∅. The set of Scott open sets of P forms a topology, called the Scott topology and denoted by σ (P). Proposition 2.1 (See [4]). If P is a continuous domain, then the approximating relation has the interpolation property (INT): x z ⇒ ∃y ∈ P such that x y z. Proposition 2.2 (See [4]). Let P be a continuous domain. Then for each x ∈ P, the set ↑x = {y ∈ P : x y} is Scott open, and these form a base for the Scott topology. A subset F of a space X is said to be irreducible, if F 6= ∅ and for any pair of closed sets F1 and F2 in X, F ⊆ F1 ∪ F2 implies that F ⊆ F1 or F ⊆ F2.A T0-space is said to be sober if every irreducible closed set is a closure of a unique point. It is known that continuous domains equipped with Scott topologies are sober (see [4, Corollary II-1.12]). A topology O(X) induces a preorder, called the specialization order which is defined by ∀u, v ∈ X, u ≤X v ⇔ u ∈ clX ({v}) ⇔ ∀U ∈ O(X), u ∈ U → v ∈ U, where clX ({v}) is the closure of the set {v} in the space (X, O(X)). The ordered set (X, ≤X ) is denoted by Ω X in short. Note that every open set of X is an upper set of Ω X and the closure of {v} is the lower set ↓ v = {x ∈ X : x ≤X v}. It is clear that (X, O(X)) is a T0-space iff ≤X is a partial order. A T0-space X is called a monotone convergence space (see [4,11]) if every subset D directed relative to the specialization order has a supremum, and the relation sup D ∈ U for any open set U of X implies D ∩ U 6= ∅. Remark 2.3 (See [4]). Each sober space is a monotone convergence space and every monotone convergence space X is a dcpo in its specialization order with O(X) ⊆ σ (Ω X). Lemma 2.4 (See [4]). For a monotone convergence space (X, O(X)) and its order of specialization, the following conditions are equivalent: (1) Ω X is a continuous domain and the topology of X is the Scott topology; (2) O(X) is a completely distributive lattice. 446 L. Xu, X. Mao / Computers and Mathematics with Applications 56 (2008) 444–452 Definition 2.5 (see [12]). Let L be a complete lattice. (1) The complete way-below relation C in L is defined for all a, b ∈ L, a C b ⇔ (∀S ⊆ L, b ≤ sup S ⇒ ∃s ∈ S, a ≤ s); (2) If a ∈ L and B ⊆ {t ∈ L : t C a} with sup B = a, then B is called an approximating complete way-below set of a in L. We will use B(a) to denote any one of the approximating complete way-below sets of a ∈ L. Lemma 2.6 (See [12]). Let L be a complete lattice. Then L is completely distributive if and only if ∀a ∈ L, a has an approximating complete way-below set. 3. Locally super-coherent topologies and characterizations of continuous domains In this section, we introduce the new concept of locally super-coherent topology and present a purely topological characterization of continuous domains. The following definition comes from [10]. Definition 3.1 (See [10]). Let (X, O(X)) be a topological space. Then a subset U is said to be super-compact if for { } ⊆ S ∈ ⊆ any family of open sets Vi i∈I with U i∈I Vi , there is i0 I such that U Vi0 . The topology O(X) is called super-coherent if it is sober and has a base of super-compact open sets. Theorem 1 of [10] says that any super-coherent space (X, O(X)) coincides with the Scott topology of a suitable algebraic domains over X. Since algebraic domains with Scott topologies are all coherent, Theorem 1 of [10], as well as Theorem 6.4 below in this paper, justifies the choice of the terminology of super-coherent.