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Dependency grammar has a long tradition in syntactic theory, dating back to at least Tesni~re's work from the thirties3 Recently, it has gained renewed attention as empirical methods in parsing are discovering the importance of relations between words (see, e.g., (Collins, 1997)), which is what dependency grammars model explicitly do, but context-free phrasestructure grammars do not. One problem that has posed an impediment to more wide-spread acceptance of dependency grammars is the fact that there is no computationally tractable version of dependency grammar which is not restricted to projective analyses. . | Pseudo-Projectivity A Polynomially Parsable Non-Projective Dependency Grammar Sylvain Kahane and Alexis Naspt and Owen Rambow TALANA ưniversité Paris 7 sk@ccr.jussieu.fr t LIA Université d Avignon alexis.nasrSlia.univ-avignon.fr ỊCoGenTex Inc. owenScogentex.com 1 Introduction Dependency grammar has a long tradition in syntactic theory dating back to at least Tesniere s work from the thirties.1 Recently it has gained renewed attention as empirical methods in parsing are discovering the importance of relations between words see e.g. Collins 1997 which is what dependency grammars model explicitly do but context-free phrasestructure grammars do not. One problem that has posed an impediment to more wide-spread acceptance of dependency grammars is the fact that there is no computationally tractable version of dependency grammar which is not restricted to projective analyses. However it is well known that there are some syntactic phenomena such as w ỉ-movement in English or clitic climbing in Romance that require non-projective analyses. In this paper we present a form of projectivity which we call pseudoprojectivity and we present a generative stringrewriting formalism that can generate pseudo-projective analyses and which is polynomially parsable. The paper is structured as follows. In Section 2 we introduce our notion of pseudoprojectivity. We briefly review a previously proposed formalization of projective dependency grammars in Section 3. In Section 4 we extend this formalism to handle pseudo-projectivity. We informally present a parser in Section 5. 2 Linear and Syntactic Order of a Sentence 2.1 Some Notation and Terminology We will use the following terminology and notation in this paper. The hierarchical order 1The work presented in this paper is collective and the order of authors is alphabetical. dominance between the nodes of a tree T will be represented with the symbol - T and Z T. Whenever they are unambiguous the notations - and will be used. When X - y we .
