Michael Zakharyaschev

Michael Zakharyaschev
Birkbeck, University of London · Department of Computer Science and Information Systems

About

269
Publications
15,006
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6,716
Citations
Citations since 2017
48 Research Items
2149 Citations
20172018201920202021202220230100200300400
20172018201920202021202220230100200300400
20172018201920202021202220230100200300400
20172018201920202021202220230100200300400
Additional affiliations
October 2005 - present
Birkbeck, University of London
Description
  • Professor of Computer Science
September 1999 - present
King's College London
Position
  • Professor of Logic and Computation
October 1998 - present
University of Leeds
Position
  • Lecturer

Publications

Publications (269)
Article
Aiming at ontology-based data access to temporal data, we design two-dimensional temporal ontology and query languages by combining logics from the (extended) DL-Lite family with linear temporal logic LTL over discrete time (Z,<). Our main concern is first-order rewritability of ontology-mediated queries (OMQs) that consist of a 2D ontology and a p...
Preprint
Our concern is the problem of determining the data complexity of answering an ontology-mediated query (OMQ) formulated in linear temporal logic LTL over (Z,<) and deciding whether it is rewritable to an FO(<)-query, possibly with some extra predicates. First, we observe that, in line with the circuit complexity and FO-definability of regular langua...
Conference Paper
In recent years, spiking neural networks (SNNs) have received extensive attention in brain-inspired intelligence due to their rich spatially-temporal dynamics, various encoding methods, and event-driven characteristics that naturally fit the neuromorphic hardware. With the development of SNNs, brain-inspired intelligence, an emerging research field...
Conference Paper
We aim to determine which temporal instance queries can be uniquely characterised by a (polynomial-size) set of positive and negative temporal data examples. We start by considering queries formulated in fragments of propositional linear temporal logic LTL that correspond to conjunctive queries (CQs) or extensions thereof induced by the until opera...
Preprint
We aim to determine which temporal instance queries can be uniquely characterised by a (polynomial-size) set of positive and negative temporal data examples. We start by considering queries formulated in fragments of propositional linear temporal logic LTL that correspond to conjunctive queries (CQs) or extensions thereof induced by the until opera...
Article
Our concern is the problem of efficiently determining the data complexity of answering queries mediated by description logic ontologies and constructing their optimal rewritings to standard database queries. Originated in ontology-based data access and datalog optimisation, this problem is known to be computationally very complex in general, with n...
Preprint
Aiming at ontology-based data access to temporal data, we design two-dimensional temporal ontology and query languages by combining logics from the (extended) DL-Lite family with linear temporal logic LTL over discrete time (Z,<). Our main concern is first-order rewritability of ontology-mediated queries (OMQs) that consist of a 2D ontology and a p...
Chapter
We prove that, similarly to known \(\textsc {PSpace}\)-completeness of recognising \(\mathsf {FO}(<)\)-definability of the language \({\boldsymbol{L}}(\mathfrak A)\) of a DFA \(\mathfrak A\), deciding both \(\mathsf {FO}(<,\equiv )\)- and \(\mathsf {FO}(<,\mathsf {MOD})\)-definability (corresponding to circuit complexity in \({\textsc {AC}^0}\) and...
Preprint
We show that deciding boundedness (aka FO-rewritability) of monadic single rule datalog programs (sirups) is 2Exp-hard, which matches the upper bound known since 1988 and finally settles a long-standing open problem. We obtain this result as a byproduct of an attempt to classify monadic `disjunctive sirups' -- Boolean conjunctive queries q with una...
Preprint
We prove that, similarly to known PSpace-completeness of recognising FO(<)-definability of the language L(A) of a DFA A, deciding both FO(<,C)- and FO(<,MOD)-definability are PSpace-complete. (Here, FO(<,C) extends the first-order logic FO(<) with the standard congruence modulo n relation, and FO(<,MOD) with the quantifiers checking whether the num...
Article
We investigate ontology-based data access to temporal data. We consider temporal ontologies given in linear temporal logic LTL interpreted over discrete time (Z,<). Queries are given in LTL or MFO(<), monadic first-order logic with a built-in linear order. Our concern is first-order rewritability of ontology-mediated queries (OMQs) consisting of a...
Chapter
We discuss recent attempts to extend the ontology-based data access (aka virtual knowledge graph) paradigm to the temporal setting. Our main aim is to understand when answering temporal ontology-mediated queries can be reduced to evaluating standard first-order queries over timestamped data and what numeric predicates and operators are required in...
Conference Paper
Traditionally, description logic has focused on representing and reasoning about classes rather than relations (roles), which has been justified by the deterioration of the computational properties if expressive role inclusions are added. The situation is even worse in the temporalised setting, where monodicity is viewed as an almost necessary cond...
Conference Paper
Aiming to understand the data complexity of answering conjunctive queries mediated by an axiom stating that a class is covered by the union of two other classes, we show that deciding their first-order rewritability is PSPACE-hard and obtain a number of sufficient conditions for membership in AC0, L, NL, and P. Our main result is a complete syntact...
Preprint
Aiming to understand the data complexity of answering conjunctive queries mediated by an axiom stating that a class is covered by the union of two other classes, we show that deciding their first-order rewritability is PSpace-hard and obtain a number of sufficient conditions for membership in AC0, L, NL, and P. Our main result is a complete syntact...
Preprint
We investigate ontology-based data access to temporal data. We consider temporal ontologies given in linear temporal logic LTL interpreted over discrete time (Z,<). Queries are given in LTL or MFO(<), monadic first-order logic with a built-in linear order. Our concern is first-order rewritability of ontology-mediated queries (OMQs) consisting of a...
Conference Paper
We investigate the data complexity of answering queries mediated by metric temporal logic ontologies under the event-based semantics assuming that data instances are finite timed words timestamped with binary fractions. We identify classes of ontology-mediated queries answering which can be done in AC0, NC1, L, NL, P, and coNP for data complexity,...
Chapter
It has recently been shown that first-order- and datalog-rewritability of ontology-mediated queries (OMQs) with expressive ontologies can be checked in NExpTime using a reduction to CSPs. In this paper, we present a case study for OMQs with Boolean conjunctive queries and a fixed ontology consisting of a single covering axiom , possibly supplemente...
Preprint
We investigate the data complexity of answering queries mediated by metric temporal logic ontologies under the event-based semantics assuming that data instances are finite timed words timestamped with binary fractions. We identify classes of ontology-mediated queries answering which can be done in AC0, NC1, L, NL, P, and coNP for data complexity,...
Preprint
Horn description logics are syntactically defined fragments of standard description logics that fall within the Horn fragment of first-order logic and for which ontology-mediated query answering is in PTime for data complexity. They were independently introduced in modal logic to capture the intersection of Horn first-order logic with modal logic....
Article
Our concern is the completeness problem for spi-logics, that is, sets of im- plications between strictly positive formulas built from propositional variables, conjunc- tion and modal diamond operators. Originated in logic, algebra and computer science, spi-logics have two natural semantics: meet-semilattices with monotone operators provid- ing Birk...
Article
Full-text available
Predictive analysis gradually gains importance in industry. For instance, service engineers at Siemens diagnostic centres unveil hidden knowledge in huge amounts of historical sensor data and use it to improve the predictive systems analysing live data. Currently, the analysis is usually done using data-dependent rules that are specific to individu...
Preprint
We investigate the problem whether two ALC ontologies are indistinguishable (or inseparable) by means of queries in a given signature, which is fundamental for ontology engineering tasks such as ontology versioning, modularisation, update, and forgetting. We consider both knowledge base (KB) and TBox inseparability. For KBs, we give model-theoretic...
Chapter
We present STypeS, a system that rewrites ontology-mediated queries with linear tuple-generating dependencies and conjunctive queries to equivalent nonrecursive datalog (NDL) queries. The main feature of STypeS is that it produces polynomial-size rewritings whenever the treewidth of the input conjunctive queries and the size of the chases for the o...
Chapter
We present STYPES, a system that rewrites ontology-mediated queries with linear tuple-generating dependencies and conjunctive queries to equivalent nonrecursive datalog (NDL) queries. The main feature of STYPES is that it produces polynomial-size rewritings whenever the treewidth of the input conjunctive queries and the size of the chases for the o...
Article
We investigate the problem whether two ALC ontologies are indistinguishable (or inseparable) by means of queries in a given signature, which is fundamental for ontology engineering tasks such as ontology versioning, modularisation, update, and forgetting. We consider both knowledge base (KB) and TBox inseparability. For KBs, we give model-theoretic...
Article
Full-text available
We propose a novel framework for ontology-based access to temporal log data using a datalog extension datalogMTL of a Horn fragment of the metric temporal logic MTL. We show that datalogMTL is ExpSpace-complete even with punctual intervals, in which case full MTL is known to be undecidable. We also prove that nonrecursive datalogMTL is PSpace-compl...
Conference Paper
We present the framework of ontology-based data access, a semantic paradigm for providing a convenient and user-friendly access to data repositories, which has been actively developed and studied in the past decade. Focusing on relational data sources, we discuss the main ingredients of ontology-based data access, key theoretical results, technique...
Chapter
We present the framework of ontology-based data access, a semantic paradigm for providing a con- venient and user-friendly access to data reposito- ries, which has been actively developed and stud- ied in the past decade. Focusing on relational data sources, we discuss the main ingredients of ontology-based data access, key theoretical results, tec...
Conference Paper
We report on our recent results in the ongoing attempts to classify conjunctive queries (CQs) \({\varvec{q}}\) according to the data complexity of answering ontology-mediated queries of the form \((\{A \sqsubseteq F \sqcup T\},{\varvec{q}})\). In particular, we present new families of path CQs for which this problem is NL-, P- or coNP-complete.
Chapter
Full-text available
We report on our experience in ontology-based data access to the Slegge database at Statoil and share the resources employed in this use case: end-user information needs (in natural language), their translations into SPARQL, the Subsurface Exploration Ontology, the schema of the Slegge database with integrity constraints, and the mappings connectin...
Conference Paper
Predictive analysis gradually gains importance in industry. For instance, service engineers at Siemens diagnostic centres unveil hidden knowledge in huge amounts of historical sensor data and use this knowledge to improve the predictive systems analysing live data. Currently, the analysis is usually done using data-dependent rules that are specific...
Article
Our concern is the completeness problem for strongly positive (SP) theories, that is, sets of implications between SP-terms built from propositional variables, conjunction and modal diamond operators. Originated in logic, algebra and computer science, SP-theories have two natural semantics: meet-semilattices with monotone operators providing Birkho...
Conference Paper
Our concern is the overhead of answering OWL 2 QL ontology-mediated queries (OMQs) in ontology-based data access compared to evaluating their underlying tree-shaped and, more generally, bounded treewidth conjunctive queries (CQs). We show that OMQs with bounded depth ontologies have nonrecursive datalog (NDL) rewritings that can be constructed and...
Chapter
Our concern is the overhead of answering OWL 2 QL ontology-mediated queries (OMQs) in ontology-based data access compared to evaluating their underlying tree-shaped and, more generally, bounded treewidth conjunctive queries (CQs). We show that OMQs with bounded depth ontologies have nonrecursive datalog (NDL) rewritings that can be constructed and...
Article
We advocate DatalogMTL - a datalog extension of a Horn fragment of the metric temporal logic MTL - as a language for ontology-based access to temporal log data. We show that DatalogMTL is ExpSpace-complete even with punctual intervals, in which case full MTL is known to be undecidable. Nonrecursive DatalogMTL turns out to be PSpace-complete for com...
Chapter
The question whether an ontology can safely be replaced by another, possibly simpler, one is fundamental for many ontology engineering and maintenance tasks. It underpins, for example, ontology versioning, ontology modularization, forgetting, and knowledge exchange. What ‘safe replacement’ means depends on the intended application of the ontology....
Article
We advocate datalogMTL, a datalog extension of a Horn fragment of the metric temporal logic MTL, as a language for ontology-based access to temporal log data. We show that datalogMTL is EXPSPACE-complete even with punctual intervals, in which case MTL is known to be undecidable. Nonrecursive datalogMTL turns out to be PSPACE-complete for combined c...
Article
Full-text available
Our concern is the overhead of answering OWL 2 QL ontology-mediated queries (OMQs) in ontology-based data access compared to evaluating their underlying tree-shaped and bounded treewidth conjunctive queries (CQs). We show that OMQs with bounded-depth ontologies have nonrecursive datalog (NDL) rewritings that can be constructed and evaluated in LOGC...
Conference Paper
We advocate datalogMTL, a datalog extension of a Horn fragment of the metric temporal logic MTL, as a language for ontology-based access to temporal log data. We show that datalogMTL is EXPSPACE-complete even with punctual intervals , in which case MTL is known to be undecidable. Non-recursive datalogMTL turns out to be PSPACE-complete for combined...
Article
Full-text available
We present a new metric temporal logic HornMTL over dense time and its datalog extension datalogMTL. The use of datalogMTL is demonstrated in the context of ontology-based data access over meteorological data. We show decidability of answering ontology-mediated queries for a practically relevant non-recursive fragment of datalogMTL. Finally, we dis...
Article
Our concern is answering ontology-mediated queries (Ο q), whereΟ is a set of linear tgds and q a conjunctive query (CQ) of bounded hypertree width. Assuming that the arity of predicates is bounded, we show that polynomial-size nonrecursive Datalog rewritings can be constructed and executed in (i) LOGCFL for OMQs with ontologies of bounded existenti...
Article
This paper reports on our ongoing work that aims at a classification of conjunctive queries q according to the data complexity of answering ontologymediated queries ({A T F}; q). We give examples of queries from the complexity classes ϵ {AC0; L; NL; P; CONP}, and obtain a few syntactical conditions for -membership and -hardness.
Article
We discuss the parameterised complexity of answering tree-shaped ontology-mediated queries (OMQs) in OWL2QL under various restrictions on their ontologies and conjunctive queries (CQs). In particular, we construct an ontology τ such that answering OMQs (τ, q) with tree-shaped CQs q is W[1]- hard if the number of leaves in q is regarded as the param...
Chapter
We advocate datalogMTL, a datalog extension of a Horn fragment of the metric temporal logic MTL, as a language for ontology-based access to temporal log data. We show that datalogMTL is EXPSPACE-complete even with punctual intervals, in which case MTL is known to be undecidable. Nonrecursive datalogMTL turns out to be PSPACE-complete for combined c...
Chapter
We discuss the use of various temporal knowledge representation formalisms for ontology-mediated query answering over temporal data. In particular, we analyse ontology and query languages based on the linear temporal logic LTL, the multi-dimensional Halpern-Shoham interval temporal logic HSn, as well as the metric temporal logic MTL. Our main focus...
Article
We give solutions to two fundamental computational problems in ontology-based data access with the W3C standard ontology language OWL 2 QL: the succinctness problem for first-order rewritings of ontology-mediated queries (OMQs), and the complexity problem for OMQ answering. We classify OMQs according to the shape of their conjunctive queries (treew...
Article
Full-text available
We show that, for OWL 2 QL ontology-mediated queries with (i) ontologies of bounded depth and conjunctive queries of bounded treewidth, (ii) ontologies of bounded depth and bounded-leaf tree-shaped conjunctive queries, and (iii) arbitrary ontologies and bounded-leaf tree-shaped conjunctive queries, one can construct and evaluate nonrecursive datalo...
Article
Full-text available
We investigate the problem whether two ALC knowledge bases are indistinguishable by queries over a given vocabulary. We give model-theoretic criteria and prove that this problem is undecidable for conjunctive queries (CQs) but decidable in 2EXPTIME for unions of rooted CQs. We also consider the problem whether two ALC TBoxes give the same answers t...
Article
We investigate the satisfiability problem for Horn fragments of the Halpern-Shoham interval temporal logic depending on the type (box or diamond) of the interval modal operators, the type of the underlying linear order (discrete or dense), and the type of semantics for the interval relations (reflexive or irreflexive). For example, we show that sat...
Article
We consider conjunctive query inseparability of description logic knowledge bases with respect to a given signature—a fundamental problem in knowledge base versioning, module extraction, forgetting and knowledge exchange. We give a uniform game-theoretic characterisation of knowledge base conjunctive query inseparability and develop worst-case opti...
Chapter
We design an extension datalogHS2 n of datalog with hyperrectangle generalisations of HalpernShoham’s modal operators on intervals and a corresponding query language. We prove that, over n-dimensional spaces comprised of Z and R, finding certain answers to datalogHS2 n queries can be reduced to standard datalog query answering. We present experimen...
Chapter
We investigate the problem whether two ALC knowledge bases are indistinguishable by queries over a given vocabulary. We give model-theoretic criteria and prove that this problem is undecid- able for conjunctive queries (CQs) but decidable in 2EXPTIME for unions of rooted CQs. We also consider the problem whether two ALC TBoxes give the same answers...
Article
We design a tractable Horn fragment of the Halpern-Shoham temporal logic and extend it to interval-based temporal description logics, instance checking in which is P-complete for both combined and data complexity.
Chapter
Aiming at ontology-based data access over temporal, in particular streaming data, we design a language of ontology-mediated queries by extending OWL 2 QL and SPARQL with temporal operators, and investigate rewritability of these queries into two-sorted first-order logic with < and PLUS over time.
Chapter
Deciding inseparability of description logic knowledge bases (KBs) with respect to conjunctive queries is fundamental for many KB engineering and maintenance tasks including versioning, module extraction, knowledge exchange and forgetting. We study the combined and data complexity of this inseparability problem for fragments of Horn-ALCHI, includin...
Chapter
We design a tractable Horn fragment of the Halpern-Shaham temporal logic and extend it to interval-based temporal description logics, instance checking in which is P-complete for both combined and data complexity.
Article
We want to understand when a given TBox T in a description logic L can be rewritten into a TBox T′ in a weaker description logic L′. Two notions of rewritability are considered: model-conservative rewritability (T′ entails T and all models of T can be expanded to models of T′) and L-conservative rewritability (T′ entails T and every L-consequence o...
Article
The language RCC8RCC8 is a widely-studied formalism for describing topological arrangements of spatial regions. The variables of this language range over the collection of non-empty, regular closed sets of n -dimensional Euclidean space, here denoted RC+(Rn)RC+(Rn), and its non-logical primitives allow us to specify how the interiors, exteriors and...
Article
HughesG. E. and CresswellM. J.. A new introduction to modal logic. Routledge, London and New York1996, x + 421 pp. - Volume 62 Issue 4 - Michael Zakharyaschev
Conference Paper
We present an extension of the ontology-based data access platform Ontop that supports answering SPARQL queries under the OWL 2 QL direct semantics entailment regime for data instances stored in relational databases. On the theoretical side, we show how any input SPARQL query, OWL 2 QL ontology and R2RML mappings can be rewritten to an equivalent S...
Chapter
This chapter gives an overview of the description logics underlying the OWL 2 Web Ontology Language and its three tractable profiles, OWL2 RL, OWL2EL and OWL 2QL. We consider the syntax and semantics of these description logics as well as main reasoning tasks and their computational complexity.We also discuss the semantical foundations for first-or...
Article
We give a solution to the succinctness problem for the size of first-order rewritings of conjunctive queries in ontology-based data access with ontology languages such as OWL 2 QL , linear Datalog±Datalog± and sticky Datalog±Datalog±. We show that positive existential and nonrecursive datalog rewritings, which do not use extra non-logical symbols (...
Article
Full-text available
We investigate the succinctness problem for conjunctive query rewritings over OWL 2QL ontologies of depth 1 and 2 by means of hypergraph programs computing Boolean functions. Both positive and negative results are obtained. We show that, over ontologies of depth 1, conjunctive queries have polynomial-size nonrecursive datalog rewritings; tree-shape...
Chapter
We discuss the celebrated Blok-Esakia theorem on the isomorphism between the lattices of extensions of intuitionistic propositional logic and the Grzegorczyk modal system. In particular, we present the original algebraic proof of this theorem found by Blok, and give a brief survey of generalisations of the Blok-Esakia theorem to extensions of intui...
Article
Full-text available
We investigate the size of first-order rewritings of conjunctive queries over OWL 2 QL ontologies of depth 1 and 2 by means of hypergraph programs computing Boolean functions. Both positive and negative results are obtained. Queries over ontologies of depth 1 have polynomial-size nonrecursive datalog rewritings; tree-shaped queries have quadratic p...
Article
We consider a new description logic ALCIr that extends ALCI with role inclusion axioms of the form R subset of QR(1)...R-m satisfying a certain regularity condition. We prove that concept satisfiability with respect to RBoxes in this logic is EXPTIME-hard. We then define a restriction ALCIr(-) of ALCIr and show that concept satisfiability with resp...
Chapter
We investigate conjunctive query inseparability of description logic knowledge bases (KBs) with respect to a given signature, a fundamental problem for KB versioning, module extraction, forgetting and knowledge exchange. We develop a game-theoretic technique for checking query inseparability of KBs expressed in fragments of Horn-ALCHI, and show a n...
Chapter
We investigate various types of query rewriting over ontologies given in the standard temporal logic LTL as well as combinations of LTL with DL-Lite logics. In particular, we consider FO(<)-rewritings that can use the temporal precedence relation, FO(<; +)-rewritings that can also employ the arithmetic predicate PLUS, and rewritings to fi�nite auto...
Conference Paper
We present the architecture and technologies underpinning the OBDA system Ontop and taking full advantage of storing data in relational databases. We discuss the theoretical foundations of Ontop: the tree-witness query rewriting, \(\mathcal{T}\)-mappings and optimisations based on database integrity constraints and SQL features. We analyse the perf...
Conference Paper
Full-text available
Ontology-based data access (OBDA) is regarded as a key ingredient of the new generation of information systems. In the OBDA paradigm, an ontology defines a high-level global schema of (already existing) data sources and provides a vocabulary for user queries. An OBDA system rewrites such queries and ontologies into the vocabulary of the data source...
Article
Full-text available
We show how the reasoning tasks of checking various versions of conservativity for the description logic DL-Litebool can be reduced to satis ability of quanti ed Boolean formulas and how o -the-shelf QBF solvers perform on a number of typical DL-Litebool ontologies.
Conference Paper
Full-text available
We introduce and investigate a number of fragments of propo- sitional temporal logic LTL over the flow of time (Z, <). The fragments are defined in terms of the available temporal operators and the structure of the clausal normal form of the temporal formulas. We determine the computational complexity of the satisfiability problem for each of the f...
Article
We design a decidable extension of the description logic SROIQ underlying the Web Ontology Language OWL2. The new logic, called SR+OIQ, supports a controlled use of role axioms whose right-hand side may contain role chains or role unions. We give a tableau algorithm for checking concept satisfiability with respect to SR+OIQ ontologies and prove its...
Article
Full-text available
Our aim is to investigate ontology-based data access over temporal data with validity time and ontologies capable of temporal conceptual modelling. To this end, we design a temporal description logic, TQL, that extends the standard ontology language OWL 2 QL, provides basic means for temporal conceptual modelling and ensures first-order rewritabili...
Article
We evaluate the performance of the OBDA system Ontop and compare it with other systems. Our experiments show that (i) the Ontop tree-witness query rewriter is fast and outperforms the competi- tors, and (ii) query evaluation in Ontop using the semantic index is as efficient as in materialisation-based systems (but without the materiali- sation over...
Article
We design a temporal description logic, TQL, that extends the standard ontology language OWL 2QL, provides basic means for temporal conceptual modelling and ensures first-order rewritability of conjunctive queries for suitably defined data instances with validity time.
Article
We use results on modal definability of first-order formulas to reduce the problem of answering conjunctive queries over knowledge bases (of any expressivity) to checking inconsistency of those knowledge bases extended with a number of ALCI concept assertions. This reduction has been shown to work for conjunctive queries without c ycles that only i...
Article
We present technologies that underpin the OBDA system Ontop and take full advantage of storing data in relational databases. We discuss the theoretical foundations of Ontop, including the tree-witness query rewriting, T -mappings and optimisations based on database integrity constraints and SQL features.
Article
We describe the architecture of the OBDA system Ontop and analyse its performance in a series of experiments. We demonstrate that, for standard ontologies, queries and data stored in relational databases, Ontop is fast, efficient and produces SQL rewritings of high quality.
Article
We settle an open question on the complexity of the following problem: given two OWL2QL TBoxes and a signature, decide whether these TBoxes return the same answers to any conjunctive query over any data formulated in the given signature. It has been known that the complexity of this problem is between PSpace and ExpTime. Here we show that the probl...
Article
Full-text available
We design temporal description logics suitable for reasoning about temporal conceptual data models and investigate their computational complexity. Our formalisms are based on DL-Lite logics with three types of concept inclusions (ranging from atomic concept inclusions and disjointness to the full Booleans), as well as cardinality constraints and ro...
Conference Paper
Full-text available
We establish connections between the size of circuits and formulas computing monotone Boolean functions and the size of first-order and nonrecursive Datalog rewritings for conjunctive queries over OWL 2 QL ontologies. We use known lower bounds and separation results from circuit complexity to prove similar results for the size of rewritings that do...
Article
Full-text available
We present a novel rewriting technique for conjunctive query answering over OWL2QL ontologies. In general, the obtained rewritings are not necessarily correct and can be of exponential size in the length of the query. We argue, however, that in most, if not all, practical cases the rewritings are correct and of polynomial size. Moreover, we prove s...
Article
Full-text available
We consider the quantifier-free languages, Bc and Bc0, obtained by augmenting the signature of Boolean algebras with a unary predicate representing, respectively, the property of being connected, and the property of having a connected interior. These languages are interpreted over the regular closed sets of n-dimensional Euclidean space (n greater...