Unni Cathrine Eiken February 2005

2.1.2.1.2 Hobbs’ Tree search algorithm
Hobbs’ tree search algorithm (Hobbs 1978, in Jurafsky and Martin 2000) is a pronoun resolution
algorithm based on syntactic tree structures of the sentences to be processed. Proceeding to
resolve the antecedent of a pronoun, the tree search algorithm processes syntactic
representations of all previous sentences in the discourse, as well as the sentence with the
pronoun to be resolved. The syntactic representations of the discourse, in combination with the
order the search of the syntactic structures is performed, to some degree represent a discourse
model and salience preferences. In its search for the antecedent of a pronoun, the algorithm
traverses syntactic trees in a left-to-right, breadth-first manner.
To find the antecedent for the pronoun han in the sentences in (2-11), syntactic tree structures of
the sentences are needed. The syntactic structures of (2-11) presented in Figure 1 and Figure 2
are generated by the NorGram grammar web version 3 . This is a more complex grammar than the
one used in Jurafsky and Martin’s outline of the algorithm (Jurafsky and Martin 2000, p. 689),
but as stated there, the algorithm to a large degree allows any choice of grammar. Since the
algorithm is based on searches in syntactic trees, and therefore relies on making assumptions
regarding the build-up of the syntactic structures, the grammar must be specified in any case. In
the following, the tree search as specified in Jurafsky and Martin (2000) will be carried through,
with the assumptions regarding the grammar that that implies. The syntactic trees (and their
labels) are included for illustrational purposes and should not be thought of as input for the
algorithm.
3 Generated at http://decentius.hit.uib.no:8010/logon/xle-mrs.xml 31/01-2005
16