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Link grammar (LG) is a theory of syntax by Davy Temperley and Daniel Sleator which builds relations between pairs of words, rather than constructing constituents in a tree-like hierarchy. There are two basic parameters: directionality and distance. Link grammar is similar to dependency grammar, but dependency grammar includes a head-dependent relationship, whereas Link Grammar makes the head-dependent relationship optional (Links need not indicate direction). Colored Multiplanar Link Grammar (CMLG) is an extension of LG allowing crossing relations between pairs of words. The relationship between words is indicated with link types, thus making the Link grammar closely related to certain categorial grammars. For example, in a subject–verb–object language like English, the verb would look left to form a subject link, and right to form an object link. Nouns would look right to complete the subject link, or left to complete the object link. In a subject–object–verb language like Persian, the verb would look left to form an object link, and a more distant left to form a subject link. Nouns would look to the right for both subject and object links. == Syntax == Rightward links are represented as a +, and leftward links with a −. Optional links are contained in curly brackets . Undesirable links are contained in any number of square brackets (). Multiple links are joined either by a conjunction & or a disjunction or. Each rule ends with a semicolon ;. The square brackets effectively give different parses a (floating-point) cost, making the most-likely parse "cheap", and the less likely parses "expensive". Costs are additive, and thus behave like the logarithm of the probability (since log-liklihoods are additive), or equivalently, somewhat like the entropy (since entropies are additive). This makes the Link Grammar compatible with machine learning techniques such as hidden Markov models and the Viterbi algorithm, because the link costs correspond to the link weights in Markov networks or Bayesian networks. Newer versions include an extension to this syntax that allows the head and dependent words to be indicated, thus enabling a more traditional dependency-grammar-like parse to be generated. Another exension simplifies the specification of parse rules for languages that have little or no restrictions on word-order, such as Lithuanian. There are also extensions to make it easier to support languages with concatenative morphologies. The Link Grammar link types can be understood to be the types in the sense of type theory. In effect, the Link Grammar can be used to model the internal language of certain (non-symmetric) compact closed categories, such as pregroup grammars. In this sense, Link Grammar appears to be isomorphic or homomorphic to some categorial grammars. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「link grammar」の詳細全文を読む スポンサード リンク
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