Polarization and abstraction of grammatical formalisms as methods for lexical disambiguation

Polarization and abstraction of grammatical formalisms as methods for lexical disambiguation Guillaume Bonfante and Bruno Guillaume and Guy Perrier LORIA - UMR 7503, Campus Scientifique, B.P. 239, F-54506 Vandœuvre l`es Nancy CEDEX {Guillaume.Bonfante, Bruno.Guillaume, Guy.Perrier}@loria.fr Abstract In the context of lexicalized grammars, we propose general methods for lexical disam- biguation based on polarization and ab- straction of grammatical formalisms. Polar- ization makes their resource sensitivity ex- plicit and abstraction aims at keeping essen- tially the mechanism of neutralization be- tween polarities. Parsing with the simpli- fied grammar in the abstract formalism can be used efficiently for filtering lexical selec- tions. Introduction There is a complexity issue if one consider ex- act parsing with large scale lexicalized gram- mars. Indeed, the number of way of associating to each word of a sentence a corresponding ele- mentary structure—a tagging of the sentence— is the product of the number of lexical entries for each word. The procedure may have an expo- nential complexity in the length of the sentence. In order to filter taggings, we can use proba- bilistic methods (Joshi and Srinivas, 1994) and keep only the most probable ones; but if we want to keep all successful taggings, we must use exact methods. Among these, one consists in abstracting information that is relevant for the filtering process, from the formalism F used for representing the concerned grammar G. In this way, we obtain a new formalism Fabs which is a simplification of F and the grammar G is translated into a grammar abs(G) in the ab- stract framework Fabs. From this, disambiguat- ing with G consists in parsing with abs(G). The abstraction is relevant if parsing eliminates a maximum of bad taggings at a minimal cost. (Boullier, 2003) uses such a method for Lexical- ized Tree Adjoining Grammars (LTAG) by ab- stracting a tree adjoining grammar into a con- text free grammar and further abstracting that one into a regular grammar. We also propose to apply abstraction but after a preprocessing polarization step. The notion of polarity comes from Categorial Grammars (Moortgat, 1996) which ground syn- tactic composition on the resource sensitivity of natural languages and it is highlighted in Inter- action Grammars (Perrier, 2003), which result from refining and making Categorial Grammars more flexible. Polarization of a grammatical formalism F consists in adding polarities to its syntactic structures to obtain a polarized formalism Fpol in which neutralization of polarities is used for controlling syntactic composition. In this way, the resource sensitivity of syntactic composition is made explicit. (Kahane, 2004) shows that many grammatical formalisms can be polarized by generalizing the system of polarities used in Interaction Grammars. To abstract a grammatical formalism, it is in- teresting to polarize it before because polarities allow original methods of abstraction. The validity of our method is based on a con- cept of morphism (two instances of which being polarization and abstraction) which character- izes how one should transport a formalism into another. In sections 1 and 2, we present the conceptual tools of …

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