Type-elimination-based reasoning for the description logic SHIQbs using decision diagrams and disjunctive datalog

We propose a novel, type-elimination-based method for reasoning in the description logic SHIQbs including DL-safe rules. To this end, we first establish a knowledge compilation method converting the terminological part of an ALCIb knowledge base into an ordered binary decision diagram (OBDD) which represents a canonical model. This OBDD can in turn be transformed into disjunctive Datalog and merged with the assertional part of the knowledge base in order to perform combined reasoning. In order to leverage our technique for full SHIQbs, we provide a stepwise reduction from SHIQbs to ALCIb that preserves satisfiability and entailment of positive and negative ground facts. The proposed technique is shown to be worst case optimal w.r.t. combined and data complexity and easily admits extensions with ground conjunctive queries.Comment: 38 pages, 3 figures, camera ready version of paper accepted for publication in Logical Methods in Computer Scienc

Hyperresolution for guarded formulae

AbstractThis paper investigates the use of hyperresolution as a decision procedure and model builder for guarded formulae. In general, hyperresolution is not a decision procedure for the entire guarded fragment. However we show that there are natural fragments of the guarded fragment which can be decided by hyperresolution. In particular, we prove decidability of hyperresolution with or without splitting for the fragment GF1− and point out several ways of extending this fragment without losing decidability. As hyperresolution is closely related to various tableaux methods the present work is also relevant for tableaux methods. We compare our approach to hypertableaux, and mention the relationship to other clausal classes which are decidable by hyperresolution

Hypertableau Reasoning for Description Logics

We present a novel reasoning calculus for the description logic SHOIQ^+---a knowledge representation formalism with applications in areas such as the Semantic Web. Unnecessary nondeterminism and the construction of large models are two primary sources of inefficiency in the tableau-based reasoning calculi used in state-of-the-art reasoners. In order to reduce nondeterminism, we base our calculus on hypertableau and hyperresolution calculi, which we extend with a blocking condition to ensure termination. In order to reduce the size of the constructed models, we introduce anywhere pairwise blocking. We also present an improved nominal introduction rule that ensures termination in the presence of nominals, inverse roles, and number restrictions---a combination of DL constructs that has proven notoriously difficult to handle. Our implementation shows significant performance improvements over state-of-the-art reasoners on several well-known ontologies

Modal Fragments of Second-Order Logic

Formaalin logiikan tutkimuskohteina ovat erilaiset muodolliset systeemit eli logiikat, joiden avulla voidaan mm. mekanisoida monenlaisia päättelyprosesseja. Eräs modernin formaalin logiikan keskeisistä tutkimusaiheista on modaalilogiikka, jossa perinteisempää logiikkaa laajennetaan nk. modaliteeteilla. Modaliteettien avulla voidaan luoda mitä erilaisimpia formaaleja systeemejä. Modaalilogiikalla onkin huomattava määrä sovelluksia aina tietojenkäsittelytieteestä ja matematiikan sekä fysiikan perusteista filosofiaan ja kielitieteisiin. Väitöskirja keskittyy modaalilogiikan nk. malliteoriaan. Tutkielmassa luokitellaan erilaisia formaalin logiikan systeemejä perustuen siihen, millaisia ominaisuuksia kyseisten systeemien avulla voidaan ilmaista. Mitä korkeampi ilmaisuvoima formaalilla järjestelmällä on, sitä hitaampaa on järjestelmän avulla suoritettava tietokoneellistettu päättely. Tutkielma käsittelee useita modaalilogiikan systeemejä; painopiste on erittäin korkean ilmaisuvoiman omaavien logiikoiden teoriassa. Tarkastelun kohteena olevat kysymykset liittyvät suoraan muuhun modaalilogiikan alan matemaattiseen tutkimukseen. Tutkielmassa mm. esitetään ratkaisu vuodesta 1983 avoinna olleeseen tekniseen kysymykseen koskien nk. toisen kertaluvun propositionaalisen modaalilogiikan alternaatiohierarkiaa.In this thesis we investigate various fragments of second-order logic that arise naturally in considerations related to modal logic. The focus is on questions related to expressive power. The results in the thesis are reported in four independent but related chapters (Chapters 2, 3, 4 and 5). In Chapter 2 we study second-order propositional modal logic, which is the system obtained by extending ordinary modal logic with second-order quantification of proposition symbols. We show that the alternation hierarchy of this logic is infinite, thereby solving an open problem from the related literature. In Chapter 3 we investigate the expressivity of a range of modal logics extended with existential prenex quantification of accessibility relations and proposition symbols. The principal result of the chapter is that the resulting extension of (a version of) Boolean modal logic can be effectively translated into existential monadic second-order logic. As a corollary we obtain decidability results for multimodal logics over various classes of frames with built-in relations. In Chapter 4 we study the equality-free fragment of existential second-order logic with second-order quantification of function symbols. We show that over directed graphs, the expressivity of the fragment is incomparable with that of first-order logic. We also show that over finite models with a unary relational vocabulary, the fragment is weaker in expressivity than first-order logic. In Chapter 5 we study the extension of polyadic modal logic with unrestricted quantification of accessibility relations and proposition symbols. We obtain a range of results related to various natural fragments of the system. Finally, we establish that this extension of modal logic exactly captures the expressivity of second-order logic

Issues of Decidability for Description Logics in the Framework of Resolution

Abstract. We describe two methods on the basis of which efficient resolution decision procedures can be developed for a range of description logics. The first method uses an ordering restriction and applies to the description logic ALB, which extends ALC with the top role, full role negation, role intersection, role disjunction, role converse, domain restriction, range restriction, and role hierarchies. The second method is based solely on a selection restriction and applies to reducts of ALB without the top role and role negation. The latter method can be viewed as a polynomial simulation of familiar tableaux-based decision procedures. It can also be employed for automated model generation. 1 Introduction Since the work of Kallick [13] resolution-based decision procedures for subclasses of first-order logic have drawn continuous attention [5,7,12]. There are two research areas where decidability issues also play a prominent role: extended modal logics and description logics [6,9,14]. Although is is not difficult to see that most of the logics under consideration can be translated into first-order logic, the exact relation to decidable subclasses of first-order logic and in particular to subclasses decidable by resolution is still under investigation. A recent important result describes a resolution decision procedure for the guarded fragment using a non-liftable ordering refinement [5]. But, the restrictions on the polarity of guards in guarded formulae are too strong to capture description logics with role negation (correspondingly, extended modal logics with relational negation). Description logics with role negation can be embedded into the class OneFree, for which a resolution decision procedure using a non-liftable ordering refinement exists [7,19]. However, this method cannot be extended easily to description logics with transitive roles. The method of this paper is based on the resolution framework of Bachmair and Ganzinger [4] which is also suitable for overcoming the problems associated with transitivity axioms, in particular, non-termination of resolution on the relational translation of certain transitive modal logics [3,8]

Issues of Decidability for Description Logics in the Framework of Resolution

. We describe two methods on the basis of which efficient resolution decision procedures can be developed for a range of description logics. The first method uses an ordering restriction and applies to the description logic ALB, which extends ALC with the top role, full role negation, role intersection, role disjunction, role converse, domain restriction, range restriction, and role hierarchies. The second method is based solely on a selection restriction and applies to reducts of ALB without the top role and role negation. The latter method can be viewed as a polynomial simulation of familiar tableaux-based decision procedures. It can also be employed for automated model generation. 1 Introduction Since the work of Kallick [13] resolution-based decision procedures for subclasses of first-order logic have drawn continuous attention [5,7,12]. There are two research areas where decidability issues also play a prominent role: extended modal logics and description logics [6,9,14]. Althoug..