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Complexity and expressiveness for formal structures in Natural Language Processing
Umeå University, Faculty of Science and Technology, Department of Computing Science. (Foundations of Language Processing)ORCID iD: 0000-0002-8722-5661
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The formalized and algorithmic study of human language within the field of Natural Language Processing (NLP) has motivated much theoretical work in the related field of formal languages, in particular the subfields of grammar and automata theory. Motivated and informed by NLP, the papers in this thesis explore the connections between expressibility – that is, the ability for a formal system to define complex sets of objects – and algorithmic complexity – that is, the varying amount of effort required to analyse and utilise such systems. Our research studies formal systems working not just on strings, but on more complex structures such as trees and graphs, in particular syntax trees and semantic graphs. The field of mildly context-sensitive languages concerns attempts to find a useful class of formal languages between the context-free and context-sensitive. We study formalisms defining two candidates for this class; tree-adjoining languages and the languages defined by linear context-free rewriting systems. For the former, we specifically investigate the tree languages, and define a subclass and tree automaton with linear parsing complexity. For the latter, we use the framework of parameterized complexity theory to investigate more deeply the related parsing problems, as well as the connections between various formalisms defining the class. The field of semantic modelling aims towards formally and accurately modelling not only the syntax of natural language statements, but also the meaning. In particular, recent work in semantic graphs motivates our study of graph grammars and graph parsing. To the best of our knowledge, the formalism presented in Paper III of this thesis is the first graph grammar where the uniform parsing problem has polynomial parsing complexity, even for input graphs of unbounded node degree.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2017. , 18 p.
Series
Report / UMINF, ISSN 0348-0542 ; 17.13
Keyword [en]
graph grammars, formal languages, natural language processing, parameterized complexity, abstract meaning representation, tree automata, deterministic tree-walking transducers, mildly context-sensitive languages, hyperedge replacement, tree adjoining languages, minimally adequate teacher
National Category
Computer Science
Identifiers
URN: urn:nbn:se:umu:diva-135014ISBN: 9789176017227 (print)OAI: oai:DiVA.org:umu-135014DiVA: diva2:1095810
Presentation
2017-05-19, MA121, MIT-huset 901 87 Umeå, Umeå, 10:15 (English)
Supervisors
Available from: 2017-05-17 Created: 2017-05-16 Last updated: 2017-05-18Bibliographically approved
List of papers
1. A bottom-up automaton for tree adjoining languages
Open this publication in new window or tab >>A bottom-up automaton for tree adjoining languages
2015 (English)Report (Other academic)
Abstract [en]

Current tree parsing algorithms for nonregular tree languages all have superlinear running times, possibly limiting their practical applicability. We present a bottom-up tree automaton that captures exactly the tree-adjoining languages in the non-deterministic case. The determinstic case captures a strict superset of the regular tree languages, while preserving running times linear in the size of the tree.

Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2015. 7 p.
Series
Report / UMINF, ISSN 0348-0542 ; 15.14
National Category
Computer Science
Identifiers
urn:nbn:se:umu:diva-134999 (URN)
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-17Bibliographically approved
2. A note on the complexity of deterministic tree-walking transducers
Open this publication in new window or tab >>A note on the complexity of deterministic tree-walking transducers
2013 (English)In: Non-Classical Models of Automata and Applications (NCMA 2013), Österreichische Computer Gesellschaft , 2013Conference paper, (Refereed)
Place, publisher, year, edition, pages
Österreichische Computer Gesellschaft, 2013
National Category
Computer Science
Research subject
Computer Science
Identifiers
urn:nbn:se:umu:diva-79716 (URN)
Conference
Non-Classical Models of Automata and Applications (NCMA 2013), August 13 - 14, 2013, Umeå, Sweden
Funder
Swedish Research Council, 621-2011-6080
Available from: 2013-08-29 Created: 2013-08-29 Last updated: 2017-05-16Bibliographically approved
3. Between a Rock and a Hard Place: Parsing for Hyperege Replacement DAG Grammars
Open this publication in new window or tab >>Between a Rock and a Hard Place: Parsing for Hyperege Replacement DAG Grammars
2016 (English)In: 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, 521-532 p.Conference 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.

Place, publisher, year, edition, pages
Springer Publishing Company, 2016
Series
Lecture Notes in Computer Science, ISSN 0302-9743 ; 9618
Keyword
Graph grammar, Hyperedge replacement, Abstract meaning representation, DAG grammar, Uniform membership problem, Parsing
National Category
Computer Science
Identifiers
urn:nbn:se:umu:diva-111984 (URN)10.1007/978-3-319-30000-9_40 (DOI)000378745100045 ()978-3-319-30000-9 (ISBN)978-3-319-29999-0 (ISBN)
Conference
10th International Conference on Language and Automata Theory and Applications (LATA 2016), Prague, Czech Republic, March 14-18, 2016
Available from: 2015-11-29 Created: 2015-11-29 Last updated: 2017-05-16Bibliographically approved
4. On the Regularity and Learnability of Ordered DAG Languages
Open this publication in new window or tab >>On the Regularity and Learnability of Ordered DAG Languages
2017 (English)In: Proceedings of the 22nd International Conference on the Implementation and Application of Automata / [ed] A. Carayol and C. Nicaud, 2017Conference paper, (Refereed)
Abstract [en]

Order-Preserving DAG Grammars (OPDGs) is a subclass of Hyper-Edge Replacement Grammars that can be parsed in polynomial time. Their associated class of languages is known as Ordered DAG Lan- guages, and the graphs they generate are characterised by being acyclic, rooted, and having a natural order on their nodes. OPDGs are useful in natural-language processing to model abstract meaning representa- tions. We state and prove a Myhill-Nerode theorem for ordered DAG languages, and translate it into a MAT-learning algorithm for the same class. The algorithm infers a minimal OPDG G for the target language in time polynomial in G and the samples provided by the MAT oracle. 

Series
Lecture Notes in Computer Science, 10329
National Category
Computer Science
Research subject
Computer Science
Identifiers
urn:nbn:se:umu:diva-135010 (URN)10.1007/978-3-319-60134-2 3 (DOI)
Conference
22nd International Conference Implementation and Application of Automata (CIAA), Université Paris-Est Marne-la-Vallée, 27-30 June 2017.
Available from: 2017-05-16 Created: 2017-05-16 Last updated: 2017-05-17
5. Investigating different graph representations of semantics
Open this publication in new window or tab >>Investigating different graph representations of semantics
2016 (English)Conference paper, (Refereed)
Abstract [en]

Combinatory Categorial Grammar is a generic approach to the mechanical understanding of language, where movement is minimised in favour of using combinators such as B (composition) and T (type lifting) to clearly define in which ways various constituents can refer to each other. Taking the tree languages induced by the syntactic derivations and connecting the various leaves linked through the semantics, one ends up with a class of graph languages. The present work aims to point out promising avenues of research in order to investigate this class, specifically in terms of similarities with other graph-based semantic representations, such as Abstract Meaning Representations (AMR), and furthermore what graph generating or recognising formalism would be most suitable to define the class characteristics.

National Category
Computer Science
Identifiers
urn:nbn:se:umu:diva-134998 (URN)
Conference
Sixth Swedish Language Technology Conference (SLTC), Umeå University, 17-18 November, 2016
Available from: 2017-05-15 Created: 2017-05-15 Last updated: 2017-05-17Bibliographically approved

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Citation style
  • apa
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  • de-DE
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  • Other locale
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Output format
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