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JOLLI
2006

A Note on the Expressive Power of Probabilistic Context Free Grammars

13 years 7 months ago
A Note on the Expressive Power of Probabilistic Context Free Grammars
We examine the expressive power of probabilistic context free grammars (PCFGs), with a special focus on the use of probabilities as a mechanism for reducing ambiguity by filtering out unwanted parses. Probabilities in PCFGs induce an ordering relation among the set of trees that yield a given input sentence. PCFG parsers return the trees bearing the maximum probability for a given sentence, discarding all other possible trees. This mechanism is naturally viewed as a way of defining a new class of tree languages. We formalize the tree language thus defined, study its expressive power, and show that the latter is beyond context freeness. While the increased expressive power offered by PCFGs helps to reduce ambiguity, we show that, in general, it cannot be decided whether a PCFG removes all ambiguities.
Gabriel G. Infante López, Maarten de Rijke
Added 13 Dec 2010
Updated 13 Dec 2010
Type Journal
Year 2006
Where JOLLI
Authors Gabriel G. Infante López, Maarten de Rijke
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