Year: 2018, 2017, 2016, 2015, 2014, 2013, 2012


  • 2018-28-02 Talk
    P501, Universität Leipzig, 11:00 - 13:00

    • Title: Eilenberg Theorems for Free
    • Stefan Milius, Friedrich-Alexander-Universität, Erlangen

    • Abstract:
      Eilenberg-type correspondences, relating varieties of languages (e.g., of finite words, infinite words, or trees) to pseudovarieties of finite algebras, form the backbone of algebraic language theory. We show that they all arise from the same recipe: one models languages and the algebras recognizing them by monads on an algebraic category, and applies a Stone-type duality. Our main contribution is a variety theorem that covers e.g. Wilke's and Pin's work on ∞-languages, the variety theorem for cost functions of Daviaud, Kuperberg, and Pin, and unifies the two categorical approaches of Bojańczyk and of Adámek et al. In addition we derive new results, such as an extension of the local variety theorem of Gehrke, Grigorieff, and Pin from finite to infinite words.


  • 2017-30-11 Talk
    APB/3027, TU Dresden, 09:20 - 10:50

    • Title: Generalized Consistent Query Answering under Existential Rules
    • Thomas Lukasiewicz, University of Oxford

    • Abstract:
      In the talk, I give an overview of recent results on generalised consistent query answering under existential rules. Previous work has proposed consistent query answering as a way to resolve inconsistencies in ontologies. In these approaches to consistent query answering, however, only inconsistencies due to errors in the underlying database are considered. In recent works, we additionally assume that ontological axioms may be erroneous, and that some database atoms and ontological axioms may not be removed to resolve inconsistencies. This problem is especially well suited in debugging mappings between distributed ontologies. We define two different semantics, one where ontological axioms as a whole are ignored to resolve an inconsistency, and one where only some of their instances are ignored. We then give a precise picture of the complexity of consistent query answering under these two semantics when ontological axioms are encoded as different classes of existential rules.

  • 2017-12-08 Talk
    APB/3027, TU Dresden, 10:30 - 11:30

    • Title: Interpolation, Amalgamation and Combination
    • Silvio Ghilardi, University of Milan

    • Abstract:
      We study the conditions under which existence of interpolants (for quantifier-free formulae) is modular, in the sense that it can be transferred from two first-order theories T1 , T2 to their combination T1 ∪ T2 . As an interesting application, we relate the combinability criterion we obtain for theories sharing a common Horn subtheory to superamalgamability conditions known from propositional logic and we use this fact to derive results concerning fusions transfer of interpolation properties in modal logic.

      This is joint work with A. Gianola.

  • 2017-12-08 Talk
    APB/3027, TU Dresden, 09:00 - 10:00

  • 2017-11-03 Talk
    APB/E009, TU Dresden, 13:00 - 14:30

    • Title: Automatic Behavior Composition
    • Sebastian Sardina, RMIT University (Melbourne, Australia)

    • Abstract:
      With computers now present in everyday devices like mobile phones, credit cards, cars and planes or places like homes, offices and factories, the trend is to build embedded complex systems from a collection of simple components. For example, a complex smart house system can be ``realised'' (i.e., implemented) by suitably coordinating the behaviours (i.e., the operational logic) of hundreds (or thousands) of simple devices and artifacts installed in the house, such as lights, blinds, game consoles, a vacuum cleaner, video cameras, audio systems, TVs, a floor cleaning robot, etc.

      The problem of automatically synthesising, that is, building, an effective coordinator controller for a given desired target system is called the behaviour composition problem and is the focus of this talk. The composition task can be recast in a variety of domains, including robot ecologies, ubiquitous robots or intelligent spaces, web-services, component-based development (CBD), agent programming, and automated planning.

      The talk will cover the standard formalization of the problem, several extensions proposed, and the main computational techniques proposed. Importantly, the behavior composition problem draws from various areas of Artificial Intelligence and Computer Science, including verification, reasoning about action, web-services, and generalized planning.

      A main reference for this work is the following one:
      Giuseppe De Giacomo, Fabio Patrizi, Sebastian Sardiña: Automatic behavior composition synthesis. Artificial Intelligence 196: 106-142 (2013)

      This is joint work done with Prof. Giuseppe De Giacomo and Dr. Fabio Patrizi (Sapienza University, Italy) and Dr. Nitin Yadav (Melbourne University, Australia).

  • 2017-04-19 Talk
    P501, Universität Leipzig, 15:00 - 16:30

    • Title: Formal methods for the verification of distributed algorithms
    • Paul Gastin, ENS Cachan

    • Abstract:
      We introduce an automata-theoretic method for the verification of distributed algorithms running on ring networks. In a distributed algorithm, an arbitrary number of processes cooperate to achieve a common goal (e.g., elect a leader, sort an array). Processes have unique identifiers (pids) from an infinite, totally ordered domain. An algorithm proceeds in synchronous rounds, each round allowing a process to perform a bounded sequence of actions such as send or receive a pid, store it in some register, and compare register contents wrt. the associated total order. An algorithm is supposed to be correct independently of the number of processes. To specify correctness properties, we introduce a logic that can reason about processes and pids. Referring to leader election, it may say that, at the end of an execution, each process stores the maximum pid in some dedicated register.

      We show that the verification problem of distributed algorithms can be reduced to satisfiability of a formula from propositional dynamic logic with loop and converse (LCPDL), interpreted over cylinders over a finite alphabet. This translation is independent of any restriction imposed on the algorithm. However, since the verification problem (and satisfiability for LCPDL) is undecidable, we propose an underapproximation technique, which bounds the number of rounds. This is an appealing approach, as the number of rounds needed by a distributed algorithm to conclude is often exponentially smaller than the number of processes. Using our reduction to LCPDL, we provide an automata-theoretic solution, reducing model checking to emptiness for alternating two-way automata on words. Overall, we show that round-bounded verification of distributed algorithms over rings is PSPACE-complete, provided the number of rounds is given in unary.

  • 2017-01-09 Talk
    P801, Universität Leipzig, 13:00 - 14:00

    • Title: Average motif counts in Markov chains of context-free graph transformation systems
    • Tobias Heindel, Universität Kopenhagen

    • Abstract:
      The dynamics of the average molecule count of a chemical species in a test tube over time is the paradigm example for motif counts in continuous time systems with discrete state space. More generally, we can consider average counts of "observable" graph motifs in Markov processes that are specified by graph transformation systems (GTS). Such average counts can be used to measure network flows, protein production in cells, and the speed of DNA-walkers.

      Computing average motif counts is typically extremely complicated if not impossible as it often amounts to solving the master equation of the underlying Markov chain with countable state space. The case is much different for the graph transformation analogue of context-free Chomsky grammars, namely context-free GTSs, aka hyper-edge replacement systems. For these systems, it is always possible to give for each motif a finite ordinary differential equation that captures the time evolution of the mean count of the motif precisely. To illustrate the analysis technique, the talk uses the preferential attachment process as example, a popular model in the context of citation networks.


  • 2016-06-22 Talk
    P502, Universität Leipzig, 15:00 - 16:00

    • Title: Parallel parsing of Operator Precedence languages
    • Matteo Pradella, Politecnico di Milano

    • Abstract:
      Operator precedence grammars, introduced by Floyd several decades ago, enjoy properties that make them attractive to face problems and to exploit technologies highly relevant in these days. An important instance is the local parsability property, i.e, the fact that any substring of a longer one can be parsed independently of its context without the risk of invalidating the partial parsing when analyzing another portion of the whole string. We exploit this distinguishing property by developing parallel algorithms and a tool, and suggest its further application to incremental analysis.

  • 2016-06-15 Talk
    APB/3027, TU Dresden, 10:00

    • Title: Mining DL ontologies
    • Ulrike SattlerUniversity of ManchesterSchool of Computer Science

    • Abstract:
      In a variety of domains and for a variety of purposes, people are using Description Logics (DL) ontologies: depending on how we count, there are hundreds or thousands of them. Quite often, these ontologies have a rather lightweight class hierarchy, and come with data in the form of an ABox or are used together in applications with external data. In the presence of this data, a naturally arising question is whether we can’t use this data to mine or learn general concept inclusion axioms, i.e., TBox axioms? And if yes, how can we do this, and how complex can these axioms be? In my talk, I will discuss these questions and describe our approach, which has been implemented in DL Miner. We aim at identifying hypotheses that capture interesting correlations in the data in the ontology. Our approach can be said to be logically and statistically sound: it works with DL’s open world assumption, and identifies suitable hypotheses by regarding this as a multi-dimensional optimisation problem (e.g., we are after hypotheses that are maximally supported and minimally brave). I will explain the setting for this problem, our approach, and our experiences with DL Miner. I will assume that the audience has a basic understanding of DLs, but this talk should also be of interest to anybody with some understanding of First Order Logic. This is joint work with Slava (Viachaslau) Sazonau.

  • 2016-06-10 Talk
    APB/1004, TU Dresden, 13:30 - 14:30

    • Title: Structural Subsumption Based Similarity Measures: applications with a medical ontology and in image object recognition
    • Boontawee Suntisrivaraporn, Sirindhorn International Institute of Technology (SIIT)

    • Abstract:
      Description Logics (DLs) are a family of logic-based knowledge representation formalisms, which can be used to develop ontologies in a formally well-founded way. The standard reasoning service of subsumption has proved indispensable in ontology design and maintenance. This checks, relative to the logical definitions in the ontology, whether one concept is more general/specific than another. When no subsumption relationship is identified, however, no information about the two concepts can be given. We introduce a similarity measure based on the structural subsumption algorithm for the DL EL and then generalize this to one for the DL ALEH. The proposed similarity measures compute a numerical degree of similarity between two concept descriptions despite not being in the subsumption relation. In this talk, two application scenarios will be discussed: the first is about new findings in SNOMED CT; whereas the second is the employment of the similarity measure in an image object recognition framework.

  • 2016-06-08 Talk
    P502, Universität Leipzig, 13:30 - 15:00

    • Title: Weighted computational models
    • George Rahonis, Aristotle University of Thessaloniki

    • Abstract:
      We present weighted computational models like automata over words and trees, and context-free grammars. We show that properties of their behaviors are closely related to the properties of the underlying weight structure. We list several open problems, especially arising in practical applications.

  • 2016-05-12 Talk
    APB/3027, TU Dresden, 13:30 - 14:30

    • Title: Inconsistency Handling in Ontology-Mediated Query Answering
    • Camille Bourgaux, Laboratoire de Recherche en Informatique at Université Paris-Sud, France

    • Abstract:
      The problem of querying description logic knowledge bases using conjunctive queries has been a major focus of recent description logic research. An important issue that arises in this context is how to handle the case in which the data is inconsistent with the ontology. Inconsistency-tolerant semantics can be used to obtain meaningful answers. The semantics AR, IAR, and brave are three well-known semantics, that rely on the notion of a repair, that is an inclusion-maximal subset of the data consistent with the ontology. I will present work on dealing with inconsistent DL-Lite knowledge bases, using these semantics. Beyond efficient query answering, we address the problems of explaining the query results, and of query-driven repairing of the data. We also consider variants of these semantics based on preferred repairs.


  • 2015-11-12 Talk
    APB/3027, TU Dresden, 13:30

    • Title: Language Equations
    • Alexander Okhotin, Ph.D., Department of Mathematics and Statistics, University of Turku, Finland

    • Abstract:
      Equations with formal languages as unknowns appear in many areas where sets of strings are used. Language equations with one-sided concatenation naturally represent finite automata, and have been used as a tool for deciding various logics. Equations involving concatenation of variables are fundamental for the theory of formal grammars; in particular, the FO(LFP) logic that subsumes all kinds of grammars can be regarded as a far-going generalization of language equations. Some further types of equations can represent computationally universal setsand beyond.

      For each kind of language equations, the typical research questions are assessing their expressive power (what languages can be represented, e.g., as a unique solution of some equation?) and determining the complexity of their decision problems (e.g., how hard is to test whether a given equation has any solutions?). In the present talk, results of this kind for various types of language equations shall be surveyed and compared to each other.

  • 2015-09-03 Talk
    Room t.b.a., Leipzig, 13:15 - 15:00

    • Title: Uniform Eilenberg Theorems: algebraic automata theory based on categories
    • PD Dr. Stefan Milius, Friedrich-Alexander-Universität Erlangen-Nürnberg

    • Abstract:
      Eilenberg's variety theorem, a centerpiece of algebraic automata theory, establishes a bijective correspondence between varieties of languages and pseudovarieties of monoids. In this talk I will show how to generalize this result from sets and boolean algebras to an abstract pair of algebraic categories. We introduce varieties of languages in a category C, and prove that they correspond to pseudovarieties of monoids in a category D that is a locally finite commutative variety, provided that C and D are dual on the level of finite objects. By suitable choices of these categories our result uniformly covers Eilenberg's theorem and three variants due to Pin, Pol?k and Reutenauer, respectively, and yields new Eilenberg-type correspondences. I will also show that the syntactic monoid of a language is generalized to the level of the category D. This allows for a uniform treatment of several notions of syntactic algebras known in the literature, including the syntactic monoids of Rabin and Scott (D = sets), the syntactic semirings of Pol?k (D = semilattices), and the syntactic associative algebras of Reutenauer (D = vector spaces).

  • 2015-07-29 Talk
    INF 1004, TU Dresden, 13:30

    • Title: Algorithms for Computing a Comprehensive Groebner Basis of a Parametric Polynomial System
    • Deepak Kapur, The University of New Mexico, Albuquerque, NM, USA

    • Abstract:
      A parametric polynomial system is a finite set of polynomials equations in which variables are classified into parameters and indeterminates. Many engineering and scientific problems can be modeled using parametric polynomial systems in which the same problem has to be analyzed for different values of parameters. Some applications include geometry theorem proving and discovery, image understanding and geometric modeling, and more recently program analysis using quantifier elimination. In 1992, Weispfenning introduced a concept of a comprehensive Groebner basis (CGB) and comprehensive Groebner system (CGS) of a parametric polynomial system which generalized Groebner basis proposed by Buchberger. A related concept, called parametric Groebner basis was proposed by Kapur in 1994, motivated by the application of image understanding. The talk will discuss recently developed efficient algorithms for computing a CGS and CGB of a parametric polynomial system. The algorithms have been implemented and successfully tried on many examples from the literature. Some open problems on this topic will also be proposed.
      This research is collaboration with Profs. Wang and Sun of the Chinese Academy of Sciences and my Ph.D. student Yiming Yang of UNM.

  • 2015-06-23 Seminar Lecture
    Felix Klein-Hörsaal P 501, Leipzig, 13:30 - 15:00

    • Title: Matrix Interpretations on Polyhedral Domains
    • Johannes Waldmann from HTWK Leipzig

    • Abstract:
      We refine the method of matrix interpretations for proving termination and complexity bounds of term rewrite systems: we restrict the interpretation domain by a system of linear inequalities. Admissibility of a restriction is shown by certificates whose validity can be expressed as a constraint program. This refinement is orthogonal to other features of matrix interpretations (complexity bounds, dependency pairs) but it can be used to improve complexity bounds, and we discuss its relation with the usable rules criterion. We present an implementation, and results of experiments.

  • 2015-05-13 Talk
    TU Dresden, APB 0005, 10:00

    • Title: Learning in Fuzzy Description Logics
    • Dr. Francesca Alessandra Lisi from University of Bari

    • Abstract:
      Incompleteness and vagueness are inherent properties of knowledge in several real-world domains and are particularly pervading in those domains where entities could be better described in natural language. The issues raised by incomplete and vague knowledge have been traditionally addressed in the field of Knowledge Representation within the stream of research devoted to fuzzy Description Logics (DLs). However, the problem of automatically managing the evolution of fuzzy DL ontologies still remains relatively unaddressed. In this talk, I present a method for inducing fuzzy GCI axioms from any crisp DL knowledge base. The method has been implemented and applied on real-world data in the tourism domain.

  • 2015-05-11 Talk
    TU Dresden, APB 3105, 14:50

    • Title: Modeling Concept Learning Problems with Second-Order Description Logics
    • Dr. Francesca Alessandra Lisi from University of Bari

    • Abstract:
      Among the inferences studied in Description Logics (DLs), induction has been paid increasing attention over the last decade. Indeed, useful non-standard reasoning tasks can be based on the inductive inference. Among them, Concept Learning is about the automated induction of a description for a given concept starting from classified instances of the concept. In this talk I present a formal characterization of Concept Learning in DLs which relies on recent results in Knowledge Representation and Machine Learning. Based on second-order DLs, it allows for modeling Concept Learning problems as constructive DL reasoning tasks where the construction of the solution to the problem may be subject to optimality criteria.

  • 2015-05-04 Talk
    Leipzig University, room P801, 15:00

    • Title: Distances for Quantitative Verification
    • Uli Fahrenberg from IRISA Rennes

    • Abstract:
      It has long been recognized in the verification community that for some applications, Boolean answers to verification problems do not suffice. Sometimes one would like to count things; sometimes one wants to verify models where quantitative information is essential and one needs the verification result to be robust under small quantitative changes; sometimes both the models and the properties to be verified are quantitative and one wants to know how far away the model is from satisfying the property. With the ever-increasing ubiquity of embedded systems, these types of quantitative verification are becoming more and more important.
      Different applications foster different types of quantitative verification, but it turns out that most of these essentially measure some type of distances between labeled transition systems. Starting in 2010, and together with several co-authors, I have developed a unifying framework which allows one to reason about such distance-based quantitative verification independently of the precise distance. This is essentially a metric theory of labeled transition systems, with infinite quantitative games as its main theoretical ingredient and general fixed-point equations for linear and branching distances as one of its main results. We have successfully applied our framework in specification theories and more broadly in software engineering and statistical language processing.

  • 2015-03-17 Talk
    Universität Leipzig, Felix Klein-Hörsaal P501, 15:15 - 17:00

    • Title: Decision Problems in Group Theory
    • Andrew Glass, Ph.D., Queens' College, Cambridge, U.K.

    • Abstract:
      It will be shown how certain group-theoretic constructions are used to study natural algorithmic questions. The Boone-Higman algebraic characterisation of precisely when a finitely generated group has soluble word problem will be discussed. All necessary algebraic and decision-theoretic background will be provided to make the talk generally accessible.


  • 2014-11-04 Talk
    Technische Universität Dresden, INF E005, 13:30

    • Title: Containment of Monadic Datalog
    • Pierre Bourhis (CNRS)

    • Abstract:
      We reconsider the problem of containment of monadic datalog (MDL) queries in unions of conjunctive queries (UCQs). Prior work has dealt with special cases, but has left the precise complexity characterization open. We begin by establishing a 2EXPTIME lower bound on the MDL/UCQ containment problem, resolving an open problem from the early 90’s. We then present a general approach for getting tighter bounds on the complexity, based on analysis of the number of mappings of queries into tree-like instances. We use the machinery to present an important case of the MDL/UCQ containment problem that is in co-NEXPTIME, and a case that is in EXPTIME. We then show that the technique can be used to get a new tight upper bound for containment of tree automata in UCQs. We show that the new MDL/UCQ upper bounds are tight.

  • 2014-06-30 Talk
    Technische Universität Dresden, INF/3027, 16:00

    • Title: Ontology-Based Data Access and Non-Uniform Constraint Satisfaction
    • Prof. Frank Wolter from University of Liverpool

    • Abstract:
      Ontology-based data access is concerned with querying incomplete data sources in the presence of domain-specific knowledge provided by an ontology. A central notion in this setting is that of an ontology-mediated query, which is a database query coupled with an ontology. In this talk, we discuss several classes of ontology-mediated queries, where the database queries are given as some form of conjunctive query and the ontologies are formulated in description logics or other relevant fragments of first-order logic, such as the guarded fragment and the unary-negation fragment.
      We then establish intimate connections between ontology-mediated queries and constraint satisfaction problems (CSPs) and their logical generalization, MMSNP formulas. These connections are exploited to obtain results regarding first-order and datalog-rewritability of ontology-mediated queries and P/NP dichotomies for ontology-mediated queries.

  • 2014-06-26Talk
    Universität Leipzig, Room A 531, 15:00

    • Title: Kleene's theorem in partial Conway theories with an application to weighted tree automata

    • Prof. Dr. Zoltan Esik from University of Szeged

    • Abstract:
      Each finite automaton may be seen as a system of fixed point equations and can thus be regarded as a morphism of a category enriched with a fixed point operation. The behavior of the automaton can be specified as another morphism of the same category. An advantage of this approach is that the same formalism is used for both the syntax and the semantics of automata. And when the fixed point operation satisfies a few simple identities, we can prove a generic Kleene's theorem applicable in several contexts including weighted tree automata.

  • 2014-05-23 PhD defense
    Technische Universität Dresden, INF/E005, 13:15

    • Title: Fuzzy Description Logics with General Concept Inclusions

    • Stefan Borgwardt from Technische Universität Dresden

    • Abstract:
      Description logics (DLs) are used to represent knowledge of an application domain and provide standard reasoning services to infer consequences of this knowledge. However, classical DLs are not suited to represent vagueness in the description of the knowledge. We consider a combination of DLs and Fuzzy Logics to address this task. In particular, we consider the t-norm-based semantics for fuzzy DLs introduced by Hájek in 2005. Since then, many tableau algorithms have been developed for reasoning in fuzzy DLs. Another popular approach is to reduce fuzzy ontologies to classical ones and use existing highly optimized classical reasoners to deal with them. However, a systematic study of the computational complexity of the different reasoning problems is so far missing from the literature on fuzzy DLs. Recently, some of the developed tableau algorithms have been shown to be incorrect in the presence of general concept inclusion axioms (GCIs). In some fuzzy DLs, reasoning with GCIs has even turned out to be undecidable. This work provides a rigorous analysis of the boundary between decidable and undecidable reasoning problems in t-norm-based fuzzy DLs, in particular for GCIs. Existing undecidability proofs are extended to cover large classes of fuzzy DLs, and decidability is shown for most of the remaining logics considered here. Additionally, the computational complexity of reasoning in fuzzy DLs with semantics based on finite lattices is analyzed. For most decidability results, tight complexity bounds can be derived.

  • 2014-05-22 Talk
    Technische Universität Dresden, INF/3027, 15:00

    • Title: Beyond DL-Lite: Big Data meets Heavier Ontologies

    • Prof. Ian Horrocks from University of Oxford (UK)

    • Abstract:
      The attractive features of DL-Lite are well known: data can be left in legacy/scalable data stores, and data complexity is low (AC0). However, these benefits come at the cost of a severely constrained ontology language. Many applications seem to require more expressive ontologies -- certainly many existing ontologies do not satisfy the relevant constraints. In this talk I will survey techniques for (empirically) scalable query answering in cases where more expressive ontologies are used, focussing on recent work on enhanced materialisation-based techniques.

  • 2014-05-12 Talk
    Universität Leipzig, Seminar room P 901 in Paulinum, Augustusplatz, 14:15

    • Title: Quantitative computation tree logic model checking based on possibility measure

    • Professor Dr. Yongming Li from Shaanxi Normal University (China)

    • Abstract:
      We study generalized possibilistic computation tree logic model checking in this paper, which is an extension of possibilistic computation logic model checking introduced by the authors. The system is modeled by generalized possibilistic Kripke structures (GPKS, in short), and the verifying property is specified by generalized possibilistic computation tree logic (GPoCTL, in short) formula. Based on generalized possibility measure, the method of generalized possibilistic computation tree logic model checking is discussed, and the corresponding algorithm and its complexity are shown in detail. By product, the comparison between PoCTL introduced previously by the authors and GPoCTL is given. Finally, a thermostat example is given to illustrate the PoCTL model-checking method. 

  • 2014-04-02 Talk
    Technische Universität Dresden, INF/3027, 14:00

    • Title: Automated Reasoning: why computational complexity isn't all that matters

    • Prof. Ulrike Sattler from University of Manchester,

    • Analysing the computational complexity of reasoning problems and designing (analysing, implementing, optimising, ...) automated reasoning procedures are well established methodologies of research into logics. In this talk, I will try to explain related questions that matter when trying to use logics in the applications they have been designed to be used in, and whose investigation is rather less well established. In particular, I will give a brief introduction into computational complexity of logical problems, and then discuss what these results tell us about the behaviour of reasoning procedures. I will also talk about other aspects of the performance of reasoning procedures, including performance homogeneity/heterogeneity and cognitive complexity. The latter plays an important role for the explanation of reasoning results.


  • 2013-07-27 PhD defense
    Technische Universität Dresden, WIL/B321, 15:00

    • Title: Relational Structure Theory - A Localisation Theory for Algebraic Structures

    • Mike Behrisch from Technische Universität Dresden

    • Abstract:
      This thesis extends a localisation theory for finite algebras to certain classes of infinite structures. Based on ideas and constructions originally stemming from Tame Congruence Theory, algebras are studied via local restrictions of their relational counterpart (Relational Structure Theory). In this respect, first those subsets are identified that are suitable for such a localisation process, i. e. that are compatible with the relational clone structure of the counterpart of an algebra. It is then studied which properties of the global algebra can be transferred to its localisations, called neighbourhoods. Thereafter, it is discussed how this process can be reversed, leading to the concept of covers. These are collections of neighbourhoods that allow information retrieval about the global structure from knowledge about the local restrictions. Subsequently, covers are characterised in terms of a decomposition equation, and connections to categorical equivalences of algebras are explored. In the second half of the thesis, a refinement concept for covers is introduced in order to find optimal, non-refinable covers, eventually leading to practical algorithms for their determination. Finally, the text establishes further theoretical foundations, e. g. several irreducibility notions, in order to ensure existence of non-refinable covers via an intrinsic characterisation, and to prove under some conditions that they are uniquely determined in a canonical sense. At last, the applicability of the developed techniques is demonstrated using two clear expository examples.

  • 2013-05-21 Seminar
    Technische Universität Dresden, Willersbau, 15:00

    • Title: The Hypergraph Transversal Problem: Applications and Complexity
    • Professor Georg Gottlob from University of Oxford

    • Abstract:
      Generating minimal transversals of a hypergraph, or, equivalently, dualizing a monotone DNF, is an important problem which has many applications in Computer Science. In this talk I will address this problem and its decisional variant, the recognition of the transversal hypergraph for another hypergraph. I will survey some results on problems which are known to be related to computing the transversal hypergraph, where I focus on problems in propositional Logic, formal concept analysis, databases, data mining, and Artificial Intelligence. I will then address the computational complexity of recognizing the transversal hypergraph. While the tractability and exact complexity of this problem have been open for over 30 years, it is known that the decision problem can be solved in quasipolynomial time, and in polynomial time with limited nondeterminism. Regarding the space complexity, it was recently shown that the problem is in quadratic logspace. I will also discuss large classes of instances on which the problem is known to be tractable.

      Speaker Bio:
      Georg Gottlob is a Professor of Informatics at Oxford University, a Fellow of St John's College, Oxford, and an Adjunct Professor at TU Wien. His interests include data extraction, database theory, graph decomposition techniques, AI, knowledge representation, logic and complexity. Gottlob has received the Wittgenstein Award from the Austrian National Science Fund, is an ACM Fellow, an ECCAI Fellow, a Fellow of the Royal Society, and a member of the Austrian Academy of Sciences, the German National Academy of Sciences, and the Academia Europaea. He chaired the Program Committees of IJCAI 2003 and ACM PODS 2000. He is the main founder of Lixto, a company that provides tools and services for web data extraction. Gottlob was rexcently awarded an ERC Advanced Investigator's Grant for the project "DIADEM: Domain-centric Intelligent Automated Data Extraction Methodology". More information on Georg Gottlob can be found on his Web page.

  • 2013-03-05 Seminar
    Technische Universität Dresden, INF/3027, 13:00

    • Title: Temporalised Description Logics for Monitoring Partially Observable Events
    • Marcel Lippmann from Technische Universität Dresden

    • Abstract:
      Daily life depends on complex hardware and software systems. In runtime monitoring, one (incompletely) observes the behaviour of the actual system in a structured way. This information is used to verify that the system has certain properties, which are formulated in a temporal logic, or to answer temporal queries to determine whether a certain situation has arisen. One approach to runtime monitoring is using temporalised Description Logics to formalise temporal properties and so-called ABoxes to capture incomplete observations of the system's behaviour.

      After a short introduction to runtime monitoring in general, we give an overview of the research field of temporalised Description Logics, and present some undecidability results and complexity results of the satisfiability problem in such logics. Moreover, we give a short introduction to query answering w.r.t. Description Logic knowledge bases. This talk concludes with some goals of the thesis and an outlook on what has been achieved so far.

  • 2013-01-10 Seminar
    Technische Universität Dresden, INF/3027, 10:00

    • Title: Reasoning with Vagueness in Description Logics using Automata and Tableaux

    • Stefan Borgwardt from Technische Universität Dresden

    • Abstract:
      Description logics (DLs) are used to represent knowledge of an application domain and provide standard reasoning services to infer consequences of this knowledge. However, classical DLs are not suited to represent uncertainty about the knowledge or vagueness in the description of the knowledge.
      In this talk, we will consider extensions of DLs that were developed to address these tasks. The focus lies on fuzzy DLs, which can be used to reason about vague concepts. We will follow the history of fuzzy DLs starting with the first reasoning procedures for simple fuzzy DLs.
      More recently, more expressive fuzzy DLs based on mathematical fuzzy logic have been proposed. Some of the basic reasoning algorithms still work in this more general setting, but so-called general concept inclusion axioms can lead to problems and even cause undecidability of reasoning in some cases.
      In the last part of the talk, the goal and methods of the thesis will be briefly described.


  • 2012-11-09 Seminar
    Technische Universität Dresden, INF/3027

    • 9:00, Modularity in Ontologies

      Dirk Walther from Universidad Politécnica de Madrid (UPM)

      Nowadays, logical theories in guise of ontologies are designed for applications in bioinformatics, medicine, geography, linguistics and other areas. They are often based on description logics with well-understood and -implemented reasoning problems and procedures. The challenges are now to provide automatic support for sharing and reuse of ontologies as well as their design and maintenance. A key concept in tackling the challenges is the notion of modularity. Recently we have seen the introduction of many related notions together with challenging technical results. This talk focusses on model-theoretic notions of modularity and reasoning problems that are relevant for modularity. Algorithms for module extraction for the description logics EL and ALC will be discussed as well as their complexity and an experimental evaluation.

    • 10:30, The Logical Difference for the Lightweight Description Logic EL

      Michel Ludwig from University of Liverpool

      We present results obtained from studying a logic-based approach to versioning of ontologies. Under this view, ontologies provide answers to queries about some vocabulary of interest. The difference between two versions of an ontology is given by the set of queries that receive different answers. We have investigated this approach for terminologies given in the description logic EL extended with role inclusions and domain/range restrictions for three distinct types of queries: subsumption, instance, and conjunctive queries. In all three cases, we present polynomial-time algorithms that decide whether two terminologies give the same answers to queries over a given vocabulary and compute a succinct representation of the difference if it is non-empty. We also present the tool CEX2.5, which is an implementation of the developed algorithms, and we analyze its performance during the experiments we have conducted with distinct versions of the SNOMED CT and the NCI ontologies.