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  • 1.
    Björklund, Henrik
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Martens, Wim
    Schwentick, Thomas
    Conjunctive query containment over trees using schema information2018In: Acta Informatica, ISSN 0001-5903, E-ISSN 1432-0525, Vol. 55, no 1, p. 17-56Article in journal (Refereed)
    Abstract [en]

    We study the containment, satisfiability, and validity problems for conjunctive queries over trees with respect to a schema. We show that conjunctive query containment and validity are 2EXPTIME -complete with respect to a schema, in both cases where the schema is given as a DTD or as a tree automaton. Furthermore, we show that satisfiability for conjunctive queries with respect to a schema can be decided in NP . The problem is NP -hard already for queries using only one kind of axis. Finally, we consider conjunctive queries that can test for equalities and inequalities of data values. Here, satisfiability and validity are decidable, but containment is undecidable, even without schema information. On the other hand, containment with respect to a schema becomes decidable again if the "larger" query is not allowed to use both equalities and inequalities.

  • 2.
    Drewes, Frank
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Hoffmann, Berthold
    University of Bremen.
    Contextual Hyperedge Replacement2015In: Acta Informatica, ISSN 0001-5903, E-ISSN 1432-0525, Vol. 52, no 6, p. 497-524Article in journal (Refereed)
    Abstract [en]

    Contextual hyperedge-replacement grammars (contextual grammars, for short) are an extension of hyperedge replacement grammars.  They have recently been proposed as a grammatical method for capturing the structure of object-oriented programs, thus serving as an alternative to the use of meta-models like UML class diagrams in model-driven software design.

    In this paper, we study the properties of contextual grammars. Even though these grammars are not context-free, one can show that they inherit several of the nice properties of hyperedge replacement grammars. In particular, they possess useful normal forms and their membership problem is in NP.

  • 3.
    Drewes, Frank
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Högberg, Johanna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Maletti, Andreas
    MAT learners for tree series: an abstract data type and two realizations2011In: Acta Informatica, ISSN 0001-5903, E-ISSN 1432-0525, Vol. 48, no 3, p. 165-189Article in journal (Refereed)
    Abstract [en]

    We propose abstract observation tables, an abstract data type for learning deterministic weighted tree automata in Angluin’s minimal adequate teacher (MAT) model, and show that every correct implementation of abstract observation tables yields a correct MAT learner. Besides the “classical” observation table, we show that abstract observation tables can also be implemented by observation trees. The advantage of the latter is that they often require fewer queries to the teacher.

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