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  • 1. Aichroth, Patrick
    et al.
    Weigel, Christian
    Kurz, Thomas
    Stadler, Horst
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Johanna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Schlegel, Kai
    Berndl, Emanuel
    Perez, Antonio
    Bowyer, Alex
    Volpini, Andrea
    MICO - MEDIA IN CONTEXT2015In: 2015 IEEE International Conference on Multimedia & Expo Workshops (ICMEW), 2015Conference paper (Refereed)
    Abstract [en]

    The abundance of digital content requires cost-effective technologies to extract the hidden meaning from media objects. However, current approaches fail to deal with the challenges related to cross-media analysis, metadata publishing, querying and recommendation that are necessary to overcome this challenge. In this paper, we describe the EU project MICO (Media in Context) which aims to provide the necessary technologies based on open-source software (OSS) core components.

  • 2.
    Andersson, Eric
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Johanna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Generating semantic graph corpora with graph expansion grammar2023In: 13th International Workshop on Non-Classical Models of Automata and Applications (NCMA 2023) / [ed] Nagy B., Freund R., Open Publishing Association , 2023, Vol. 388, p. 3-15Conference paper (Refereed)
    Abstract [en]

    We introduce LOVELACE, a tool for creating corpora of semantic graphs.The system uses graph expansion grammar as  a representational language, thus allowing users to craft a grammar that describes a corpus with desired properties. When given such grammar as input, the system generates a set of output graphs that are well-formed according to the grammar, i.e., a graph bank.The generation process can be controlled via a number of configurable parameters that allow the user to, for example, specify a range of desired output graph sizes.Central use cases are the creation of synthetic data to augment existing corpora, and as a pedagogical tool for teaching formal language theory. 

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  • 3.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Henrik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Algorithmic properties of Millstream systems2010In: Developments in Language Theory: 14th International Conference, DLT 2010 / [ed] Sheng Yu, Springer Berlin/Heidelberg, 2010, p. 54-65Conference paper (Refereed)
    Abstract [en]

    Millstream systems have recently been proposed as a formalization of the linguistic idea that natural language should be described as a combination of different modules related by interfaces. In this paper we investigate algorithmic properties of Millstream systems having regular tree grammars as modules and MSO logic as interface logic. We focus on the so-called completion problem: Given trees generated by a subset of the modules, can they be completed into a valid configuration of the Millstream system?

  • 4.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Millstream Systems2009Report (Other academic)
    Abstract [en]

    We introduce Millstream systems, a mathematical framework in the tradition of the Theory of Computation that uses logic to formalize the interfaces between different aspects of language, the latter being described by any number of independent modules. Unlike other approaches that serve a similar goal, Millstream systems neither presuppose nor establish a particular linguistic theory or focus, but can be instantiated in various ways to accomodate different points of view.

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  • 5.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Millstream Systems: a formal model for linking language modules by interfaces2010In: Proc. ACL 2010 Workshop on Applications of Tree Automata in Natural Language Processing (ATANLP 2010), The Association for Computer Linguistics , 2010Conference paper (Refereed)
  • 6.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, FrankUmeå University, Faculty of Science and Technology, Department of Computing Science.
    Proceedings of Umeå's 16th student conference in computing science: USCCS 20132013Conference proceedings (editor) (Other academic)
  • 7.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, FrankUmeå University, Faculty of Science and Technology, Department of Computing Science.Freund, RudolfTechnical University Vienna.Otto, FriedrichTechnical University Kassel.
    Fifth Workshop on Non-Classical Models for Automata and Applications - NCMA 2013, Umeå, Sweden, August 13 - August 14, 2013, Proceedings2013Conference proceedings (editor) (Refereed)
  • 8.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Hellström, Thomas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Grammatical Inference of Graph Transformation Rules2015In: Proceedings of the 7th Workshop on Non-Classical Modelsof Automata and Applications (NCMA 2015), Austrian Computer Society , 2015, p. 73-90Conference paper (Refereed)
  • 9.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, FrankUmeå University, Faculty of Science and Technology, Department of Computing Science.Hirvensalo, MikaOtto, Friedrich
    Fundamenta Informaticae: special issue2015Conference proceedings (editor) (Other academic)
  • 10.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jürgensen, Helmut
    Department of Computer Science, The University of Western Ontario, London, Canada.
    van der Merwe, Brink
    Department of Computer Science, Stellenbosch University, South Africa.
    Correct readers for the incremental construction of Millstream configurations by graph transformation2012Report (Other academic)
    Abstract [en]

    Millstream systems have been proposed as a non-hierarchical method for modelling natural language. Millstream congurations represent and connect multiple structural aspects of sentences. We present a method by which the Millstream congurations corresponding to a sentence are constructed. The construction is incremental, that is, it proceeds as the sentence is being read and is complete when the end of the sentence is reached. It is based on graph transformations and a lexicon which associates words with rules for the graph transformations.

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    Correct Readers for the Incremental Construction of Millstream Configurations by Graph Transformation
  • 11.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jürgensen, Helmut
    The University of Western Ontario.
    van der Merwe, Brink
    University of Stellenbosch.
    Correct Readers for the Incremental Construction of Millstream Configurations by Graph TransformationManuscript (preprint) (Other academic)
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    fulltext
  • 12.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jürgensen, Helmut
    Department of Computer Science, Western University, London, Canada.
    van der Merwe, Brink
    Department of Computer Science, Stellenbosch University, South Africa.
    Graph transformation for incremental natural language analysis2014In: Theoretical Computer Science, ISSN 0304-3975, E-ISSN 1879-2294, Vol. 531, p. 1-25Article in journal (Refereed)
    Abstract [en]

    Millstream systems have been proposed as a non-hierarchical method for modelling natural language. Millstream configurations represent and connect multiple structural aspects of sentences. We present a method by which the Millstream configurations corresponding to a sentence are constructed. The construction is incremental, that is, it proceeds as the sentence is being read and is complete when the end of the sentence is reached. It is based on graph transformations and a lexicon which associates words with graph transformation rules that implement the incremental construction process.

    Download full text (pdf)
    fulltext
  • 13.
    Bensch, Suna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jürgensen, Helmut
    Middlesex College, The University of Western Ontario.
    van der Merwe, Brink
    Department of Mathematical Sciences, University of Stellenbosch.
    Incremental Construction of Millstream Configurations Using Graph Transformation2011In: Proceedings of the 9th International Workshop on Finite State Methods and Natural Language Processing, Stroudsburg: Association for Computational Linguistics , 2011, p. 93-97Conference paper (Refereed)
  • 14.
    Berglund, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank (Contributor)
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Characterizing Non-Regularity2014Report (Other academic)
    Abstract [en]

    This paper considers a characterization of the context-free non-regular languages, conjecturing that there for all such languages exists a fixed string thatcan be pumped to exhibit infinitely many equivalence classes. A proof is given only for a special case, but the general statement is conjectured to hold. The conjecture is then shown to imply that the shuffle of two context-free languagesis not context-free.

  • 15.
    Berglund, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Henrik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    On the Parameterized Complexity of Linear Context-Free Rewriting Systems2013In: Proceedings of the 13th Meeting on the Mathematics of Language (MoL 13), Association for Computational Linguistics, 2013, p. 21-29Conference paper (Other academic)
    Abstract [en]

    We study the complexity of uniform membership for Linear Context-Free RewritingSystems, i.e., the problem where we aregiven a string w and a grammar G and areasked whether w ∈ L(G). In particular,we use parameterized complexity theoryto investigate how the complexity dependson various parameters. While we focusprimarily on rank and fan-out, derivationlength is also considered.

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    On the Parameterized Complexity of Linear Context-Free Rewriting Systems
  • 16.
    Berglund, Martin
    et al.
    Stellenbosch University.
    Björklund, Henrik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Single-Rooted DAGs in Regular DAG Languages: Parikh Image and Path Languages2017In: Proceedings of the 13th International Workshop on Tree Adjoining Grammars and Related Formalisms (TAG+13) / [ed] M. Kuhlmann, T. Scheffler, Association for Computational Linguistics , 2017, p. 94-101Conference paper (Refereed)
    Abstract [en]

    In a recent survey (Drewes, 2017) of results on DAG automata  some open problems are formulated for the case where the  DAG language accepted by a DAG automaton A is restricted to DAGs with a single root, denoted by L(A)u. Here we consider each of  those problems, demonstrating that: (i) the finiteness  of L(A)u is decidable, (ii) the path languages of L(A)u can be characterized in  terms of the string languages accepted by partially blind  multicounter automata, and  (iii) the Parikh image of L(A)u is semilinear.

  • 17.
    Berglund, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Henrik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    van der Merwe, Brink
    Stellenbosch University, South Africa.
    Watson, Bruce
    Stellenbosch University, South Africa.
    Cuts in Regular Expressions2013In: Developments in Language Theory: 17th International Conference, DLT 2013, Marne-la-Vallée, France, June 18-21, 2013. Proceedings / [ed] Marie-Pierre Béal, Olivier Carton, Springer Berlin/Heidelberg, 2013, p. 70-81Conference paper (Refereed)
    Abstract [en]

    Most software packages with regular expression matching engines offer operators that extend the classical regular expressions, such as counting, intersection, complementation, and interleaving. Some of the most popular engines, for example those of Java and Perl, also provide operators that are intended to control the nondeterminism inherent in regular expressions. We formalize this notion in the form of the cut and iterated cut operators. They do not extend the class of languages that can be defined beyond the regular, but they allow for exponentially more succinct representation of some languages. Membership testing remains polynomial, but emptiness testing becomes PSPACE-hard. 

  • 18.
    Berglund, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    On the complexity of variants of the k Best strings problem2010In: Proceedings of the Prague stringology conference 2010, dblp , 2010, p. 76-88Conference paper (Refereed)
    Abstract [en]

    We investigate the problem of extracting the k best strings from a nondeterministic weighted automaton over a semiring S. This problem, which has been considered earlier in the literature, is more difficult than extracting the k best runs, since distinct runs may not correspond to distinct strings. Unsurprisingly, the computational complexity of the problem depends on the semiring S used. We study three different cases, namely the tropical and complex tropical semirings, and the semiring of positive real numbers. For the first case, we establish a polynomial algorithm. For the second and third cases, NP-completeness and undecidability results are shown.

  • 19.
    Berglund, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    On the complexity of variants of the k best strings problem2010In: Proc. Prague Stringology Conference 2010 / [ed] M. Balík, J. Holub, 2010Conference paper (Refereed)
  • 20.
    Berglund, Martin
    et al.
    Stellenbosch University.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Merwe, Brink van der
    University of Stellenbosch.
    On Regular Expressions with Backreferences and Transducers2018In: Tenth Workshop on Non-Classical Models of Automata and Applications (NCMA 2018) / [ed] Rudolf Freund, Michal Hospodár, Galina Jirásková, Giovanni Pighizzini, Österreichische Computer Gesellschaft , 2018, p. 49-64Conference paper (Refereed)
    Abstract [en]

    Modern regular expression matching software features many extensions, some general, while some are very narrowly specified. Here we consider the generalization of adding a class of operators which can be described by, e.g. finite-state transducers. Combined with backreferences, they enable new classes of languages to be matched. The addition of finite-state transducers is shown to make membership testing undecidable. Following this result, we study the complexity of membership testing for various restricted cases of the model.

  • 21.
    Berglund, Martin
    et al.
    Stellenbosch University.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Merwe, Brink van der
    University of Stellenbosch.
    The output size problem for string-to-tree transducers2018In: Journal of Automata, Languages and Combinatorics, Vol. 23, no 1-3, p. 19-38Article in journal (Refereed)
    Abstract [en]

    The output size problem, for a string-to-tree transducer, is to determine the asymptotic behavior of the function describing the maximum size of output trees, with respect to the length of input strings. We show that the problem to determine, for a given regular expression, the worst-case matching time of a backtracking regular expression matcher, can be reduced to the output size problem. The latter can, in turn, be solved by determining the degree of ambiguity of a non-deterministic finite automaton.

  • 22.
    Berglund, Martin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    van der Merwe, Brink
    University of Stellenbosch, South Africa.
    Analyzing catastrophic backtracking behavior in practical regular expression matching2014In: Proceedings 14th international conference on automata and formal languages (AFL 2014) / [ed] Zoltán Ésik; Zoltán Fülöp, Open Publishing Association , 2014, p. 109-123Conference paper (Refereed)
    Abstract [en]

    We consider in some detail how regular expression matching happens in Java, as a popular representative of the category of regex-directed matching engines. We extract a slightly idealized algorithm for this scenario. Next we define an automata model which captures all the aspects needed to perform matching, of the Java style, in a formal way. Finally, two types of static analysis, which take a regular expression and tells whether there exists a family of strings which make Java-style matching run in exponential time, are done.

  • 23.
    Björklund, Henrik
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Ericson, Petter
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Between a Rock and a Hard Place: Parsing for Hyperege Replacement DAG Grammars2016In: Proc. 10th International Conference on Language and Automata Theory and Applications (LATA 2016) / [ed] A.H. Dediu, J. Janoušek, C. Martín-Vide, and B. Truthe, Springer Publishing Company, 2016, Vol. 9618, p. 521-532Conference paper (Refereed)
    Abstract [en]

    Motivated by applications in natural language processing, we study the uniform membership problem for hyperedge-replacement grammars that generate directed acyclic graphs. Our major result is a low-degree polynomial-time algorithm that solves the uniform membership problem for a restricted type of such grammars. We motivate the necessity of the restrictions by two different NP-completeness results.

  • 24.
    Björklund, Henrik
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Ericson, Petter
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Parsing Weighted Order-Preserving Hyperedge Replacement Grammars2019In: Proceedings of the 16th Meeting on the Mathematics of Language / [ed] F. Drewes, P. de Groote, G. Penn, Association for Computational Linguistics, 2019, p. 1-11Conference paper (Refereed)
    Abstract [en]

    We introduce a weighted extension of the recently proposed notion oforder-preserving hyperedge-replacement grammars and prove that the weightof a graph according to such a weighted graph grammar can be computeduniformly in quadratic time (under assumptions made precise in the paper).

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    fulltext
  • 25.
    Björklund, Henrik
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Ericson, Petter
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Parsing Weighted Order-Preserving Hyperedge Replacement Grammars2018Report (Other academic)
    Abstract [en]

    We introduce a weighted extension of the recently proposed notion of order-preserving hyperedge-replacement grammars and prove that the weight of a graph according to such a weighted graph grammar can be computed uniformly in quadratic time (under assumptions made precise in the paper).

    Download full text (pdf)
    fulltext
  • 26.
    Björklund, Henrik
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Ericson, Petter
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Starke, Florian
    Faculty of Computer Science, TU Dresden.
    Uniform Parsing for Hyperedge Replacement Grammars2018Report (Other academic)
    Abstract [en]

    It is well known that hyperedge-replacement grammars can generate NP-complete graph languages even under seemingly harsh restrictions. This means that the parsing problem is difficult even in the non-uniform setting, in which the grammar is considered to be fixed rather than being part of the input. Little is known about restrictions under which truly uniform polynomial parsing is possible. In this paper we propose a low-degree polynomial-time algorithm that solves the uniform parsing problem for a restricted type of hyperedge-replacement grammars which we expect to be of interest for practical applications.

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    fulltext
  • 27.
    Björklund, Henrik
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Ericson, Petter
    Umeå University, Faculty of Science and Technology, Department of Computing Science. Digital and Cognitive Musicology Lab, Ecole Polytechnique F´ed´erale de Lausanne, Switzerland.
    Starke, Florian
    Faculty of Computer Science, TU Dresden, Germany.
    Uniform Parsing for Hyperedge Replacement Grammars2021In: Journal of computer and system sciences (Print), ISSN 0022-0000, E-ISSN 1090-2724, Vol. 118, p. 1-27Article in journal (Refereed)
    Abstract [en]

    It is well known that hyperedge-replacement grammars can generate NP-complete graph languages even under seemingly harsh restrictions. This means that the parsing problem is difficult even in the non-uniform setting, in which the grammar is considered to be fixed rather than being part of the input. Little is known about restrictions under which truly uniform polynomial parsing is possible. In this paper we propose a low-degree polynomial-time algorithm that solves the uniform parsing problem for a restricted type of hyperedge-replacement grammars which we expect to be of interest for practical applications.

    Download full text (pdf)
    fulltext
  • 28.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Cohen, Shay B.
    University of Edinburgh, United Kingdom.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Satta, Giorgio
    University of Padova, Italy.
    Bottom-up unranked tree-to-graph transducers for translation into semantic graphs2021In: Theoretical Computer Science, ISSN 0304-3975, E-ISSN 1879-2294, Vol. 870, p. 3-28Article in journal (Refereed)
    Abstract [en]

    We develop a finite-state transducer for translating unranked trees into general graphs. This work is motivated by recent progress in semantic parsing for natural language, where sentences are first mapped into tree-shaped syntactic representations, and then these trees are translated into graph semantic representations. We investigate formal properties of our tree-to-graph transducers and develop a polynomial time algorithm for translating a weighted language of input trees into a packed representation, from which best-score graphs can be efficiently recovered.

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    fulltext
  • 29.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Cohen, Shay B.
    University of Edinburgh.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Satta, Giorgio
    University of Padova.
    Bottom-up unranked tree-to-graph transducers for translation into semantic graphs2019In: Proceedings of the 14th International Conference on Finite-State Methods and Natural Language Processing / [ed] Heiko Vogler; Andreas Maletti, Association for Computational Linguistics, 2019, p. 7-17, article id W19-3104Conference paper (Refereed)
    Abstract [en]

    We propose a formal model for translating unranked syntactic trees, such as dependency trees, into semantic graphs. These tree-to-graph transducers can serve as a formal basis of transition systems for semantic parsing which recently have been shown to perform very well, yet hitherto lack formalization. Our model features "extended" rules and an arc-factored normal form, comes with an efficient translation algorithm, and can be equipped with weights in a straightforward manner.

  • 30.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Dahlgren Lindström, Adam
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Bridging Perception, Memory, and Inference through Semantic Relations2021In: Proceedings of the 2021 Conference on Empirical Methods in Natural Language Processing, Association for Computational Linguistics (ACL) , 2021, p. 9136-9142Conference paper (Refereed)
    Abstract [en]

    There is a growing consensus that surface form alone does not enable models to learn meaning and gain language understanding. This warrants an interest in hybrid systems that combine the strengths of neural and symbolic methods. We favour triadic systems consisting of neural networks, knowledge bases, and inference engines. The network provides perception, that is, the interface between the system and its environment. The knowledge base provides explicit memory and thus immediate access to established facts. Finally, inference capabilities are provided by the inference engine which reflects on the perception, supported by memory, to reason and discover new facts. In this work, we probe six popular language models for semantic relations and outline a future line of research to study how the constituent subsystems can be jointly realised and integrated.

  • 31.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Comparative Evaluation of the Efficiency of N-Best Algorithms on Language DataManuscript (preprint) (Other academic)
    Abstract [en]

    The N-best extraction problem consists in selecting the N highest ranking hypotheses from a set of hypotheses, with respect to a given ranking system. In our setting, the hypotheses and ranking are jointly represented by a weighted tree automaton (wta) over the tropical semiring: the hypotheses are trees, or runs on trees, and the ranking is decided by the weight assigned to them. In previous work, we presented an algorithm for N-best extraction that combines techniques to restrict the search space, and proved it to be correct and efficient. The algorithm is now implemented in the software Betty, allowing us to complement the deductive study with an empirical investigation.  In particular, we compare our algorithm to the state-of-the-art algorithm for extracting the N best runs, implemented in in the software toolkit Tiburon. The data sets used in the experiments are wtas resulting from real-world natural language processing tasks, as well as artificially created wtas with varying degrees of nondeterminism. We find that Betty outperforms Tiburon on all tested data sets.

  • 32.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Comparison of Two N-Best Extraction Methods for Weighted Tree Automata2018In: Implementation and Application of Automata: 23rd International Conference, CIAA 2018, Charlottetown, PE, Canada, July 30 – August 2, 2018, Proceedings, Springer, 2018, p. 197-108Conference paper (Refereed)
    Abstract [en]

    We conduct a comparative study of two state-of-the-art al- gorithms for extracting the N best trees from a weighted tree automaton (wta). The algorithms are Best Trees, which uses a priority queue to structure the search space, and Filtered Runs, which is based on an algorithm by Huang and Chiang that extracts N best runs, implemented as part of the Tiburon wta toolkit. The experiments are run on four data sets, each consisting of a sequence of wtas of increasing sizes. Our conclusion is that Best Trees can be recommended when the input wtas exhibit a high or unpredictable degree of nondeterminism, whereas Filtered Runs is the better option when the input wtas are large but essentially deterministic.

  • 33.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Faster Computation of N-Best Lists for Weighted Tree AutomataManuscript (preprint) (Other academic)
    Abstract [en]

    We show that a previously proposed algorithm for the N-best trees problem – not to be confused with the easier N-best runs problem – can be made more efficient by changing how it arranges and explores the search space. Given an integer N and a weighted tree automaton (wta) M over the tropical semiring, the algorithm computes N trees of minimal weight with respect to M. Compared to the original algorithm, the modifications increase the laziness of the evaluation strategy, which makes the new algorithm asymptotically more efficient than its predecessor. 

  • 34.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Finding the N Best Vertices in an Infinite Weighted Hypergraph2017In: Theoretical Computer Science, ISSN 0304-3975, E-ISSN 1879-2294, Vol. 682, p. 78p. 30-41Article in journal (Refereed)
    Abstract [en]

    We propose an algorithm for computing the N best vertices in a weighted acyclic hypergraph over a nice semiring. A semiring is nice if it is finitely-generated, idempotent, and has 1 as its minimal element. We then apply the algorithm to the problem of computing the N best trees with respect to a weighted tree automaton, and complement theoretical correctness and complexity arguments with experimental data. The algorithm has several practical applications in natural language processing, for example, to derive the N most likely parse trees with respect to a probabilistic context-free grammar. 

  • 35.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Generation and polynomial parsing of graph languages with non-structural reentrancies2023In: Computational linguistics - Association for Computational Linguistics (Print), ISSN 0891-2017, E-ISSN 1530-9312, Vol. 49, no 4, p. 841-882Article in journal (Refereed)
    Abstract [en]

    Graph-based semantic representations are popular in natural language processing (NLP), where it is often convenient to model linguistic concepts as nodes and relations as edges between them. Several attempts have been made to find a generative device that is sufficiently powerful to describe languages of semantic graphs, while at the same allowing efficient parsing. We contribute to this line of work by introducing graph extension grammar, a variant of the contextual hyperedge replacement grammars proposed by Hoffmann et al. Contextual hyperedge replacement can generate graphs with non-structural reentrancies, a type of node-sharing that is very common in formalisms such as abstract meaning representation, but which context-free types of graph grammars cannot model. To provide our formalism with a way to place reentrancies in a linguistically meaningful way, we endow rules with logical formulas in counting monadic second-order logic. We then present a parsing algorithm and show as our main result that this algorithm runs in polynomial time on graph languages generated by a subclass of our grammars, the so-called local graph extension grammars.

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  • 36.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Improved N-Best Extraction with an Evaluation on Language Data2022In: Computational linguistics - Association for Computational Linguistics (Print), ISSN 0891-2017, E-ISSN 1530-9312, Vol. 48, no 1, p. 119-153Article in journal (Refereed)
    Abstract [en]

    We show that a previously proposed algorithm for the N-best trees problem can be made more efficient by changing how it arranges and explores the search space. Given an integer N and a weighted tree automaton (wta) M over the tropical semiring, the algorithm computes N trees of minimal weight with respect to M. Compared with the original algorithm, the modifications increase the laziness of the evaluation strategy, which makes the new algorithm asymptotically more efficient than its predecessor. The algorithm is implemented in the software BETTY, and compared to the state-of-the-art algorithm for extracting the N best runs, implemented in the software toolkit TIBURON. The data sets used in the experiments are wtas resulting from real-world natural language processing tasks, as well as artificially created wtas with varying degrees of nondeterminism. We find that BETTY outperforms TIBURON on all tested data sets with respect to running time, while TIBURON seems to be the more memory-efficient choice.

    Download full text (pdf)
    fulltext
  • 37.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    On the N best problem for hypergraphs2016In: / [ed] A. Maletti, 2016Conference paper (Other academic)
    Abstract [en]

    We propose an algorithm for computing the $N$ best roots of a weighted hypergraph, in which the weight function is given over an idempotent and multiplicatively monotone semiring. We give a set of conditions that ensures that the weight function is well-defined and that solutions exist. Under these conditions, we prove that the proposed algorithm is correct.  This generalizes a previous result for weighted tree automata, and in doing so, broadens the practical applications.

  • 38.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jonsson, Anna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Polynomial Graph Parsing with Non-Structural ReentranciesManuscript (preprint) (Other academic)
    Abstract [en]

    Graph-based semantic representations are valuable in natural language processing, where it is often simple and effective to represent linguistic concepts as nodes, and relations as edges between them. Several attempts has been made to find a generative device that is sufficiently powerful to represent languages of semantic graphs, while at the same allowing efficient parsing. We add to this line of work by introducing graph extension grammar, which consists of an algebra over graphs together with a regular tree grammar that generates expressions over the operations of the algebra. Due to the design of the operations, these grammars can generate graphs with non-structural reentrancies, a type of node-sharing that is excessively common in formalisms such as abstract meaning representation, but for which existing devices offer little support. We provide a parsing algorithm for graph extension grammars, which is proved to be correct and run in polynomial time. 

  • 39.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Mollevik, Iris
    Towards Semantic Representations with a Temporal Dimension2020Conference paper (Refereed)
    Abstract [en]

    We outline the initial ideas for a representational framework for capturing temporal aspects in semantic parsing of multimodal data.As a starting point, we take the Abstract Meaning Representations of Banarescu et al. andpropose a way of extending them to coversequential progressions of events. The firstmodality to be considered is text, but the long-term goal is to also incorporate informationfrom visual and audio modalities, as well ascontextual information.

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    fulltext
  • 40.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Satta, Giorgio
    University of Padova, Italy.
    Z-Automata for Compact and Direct Representation of Unranked Tree Languages2019In: Implementation and Application of Automata: 24th International Conference, CIAA 2019 Košice, Slovakia, July 22–25, 2019, Proceedings / [ed] Michal Hospodár, Galina Jirásková, Springer, 2019, p. 83-94Conference paper (Refereed)
    Abstract [en]

    Unranked tree languages are valuable in natural language processing for modelling dependency trees. We introduce a new type of automaton for unranked tree languages, called Z-automaton, that is tailored for this particular application. The Z-automaton offers a compact form of representation, and unlike the closely related notion of stepwise automata, does not require a binary encoding of its input. We establish an arc-factored normal form, and prove the membership problem of Z-automata in normal form to be in O(mn), where m is the size of the transition table of the Z-automaton and n is the size of the input tree.

  • 41.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Zechner, Niklas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    An efficient best-trees algorithm for weighted tree automata over the tropical semiring2015In: Proc. 9th International Conference on Language and Automata Theory and Applications / [ed] Adrian-Horia Dediu, Enrico Formenti, Carlos Martín-Vide, and Bianca Truthe, Springer, 2015, p. 97-108Conference paper (Refereed)
    Abstract [en]

    We generalise a search algorithm by Mohri and Riley from strings to trees. The original algorithm takes as input a weighted automaton M over the tropical semiring, together with an integer N, and outputs N strings of minimal weight with respect to M. In our setting, M defines a weighted tree language, again over the tropical semiring, and the output is a set of N trees with minimal weight. We prove that the algorithm is correct, and that its time complexity is a low polynomial in N and the relevant size parameters of M.

  • 42.
    Björklund, Johanna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Zechner, Niklas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Efficient enumeration of weighted tree languages over the tropical semiring2019In: Journal of computer and system sciences (Print), ISSN 0022-0000, E-ISSN 1090-2724, Vol. 104, p. 78p. 119-130Article in journal (Refereed)
    Abstract [en]

    We generalise a search algorithm by Mohri and Riley from strings to trees. The original algorithm takes as input a nondeterministic weighted automaton M over the tropical semiring and an integer N, and outputs N strings of minimal weight with respect to M. In our setting, M is a weighted tree automaton, again over the tropical semiring, and the output is a set of N trees with minimal weight in this language. We prove that the algorithm is correct, and that its time complexity is a low polynomial in N and the relevant size parameters of M. 

  • 43.
    Blum, Johannes
    et al.
    University of Würzburg.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Language Theoretic Properties of Regular DAG Languages2019In: Information and Computation, ISSN 0890-5401, E-ISSN 1090-2651, Vol. 265, p. 78p. 57-76Article in journal (Refereed)
    Abstract [en]

    We study sets of directed acyclic graphs, called regular DAG languages, which are accepted by a recently introduced type of DAG automata motivated by current developments in natural language processing. We prove (or disprove) closure properties, establish pumping lemmata, characterize finite regular DAG languages, and show that "unfolding" turns regular DAG languages into regular tree languages, which implies a linear growth property and the regularity of the path languages of regular DAG languages. Further, we give polynomial decision algorithms for the emptiness and finiteness problems, and show that deterministic DAG automata can be minimized and tested for equivalence in polynomial time.

  • 44.
    Blum, Johannes
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Drewes, Frank
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Properties of Regular DAG Languages2016In: Proc. 10th International Conference on Language and Automata Theory and Applications (LATA 2016) / [ed] A.H. Dediu, J. Janoušek, C. Martín-Vide, and B. Truthe, Springer Publishing Company, 2016, p. 427-438Conference paper (Refereed)
    Abstract [en]

    A DAG is a directed acyclic graph. We study the properties of DAG automata and their languages, called regular DAG languages. In particular, we prove results resembling pumping lemmas and show that the finiteness problem for regular DAG languages is in P.

  • 45.
    Chiang, David
    et al.