8,743 research outputs found
Multiple Context-Free Tree Grammars: Lexicalization and Characterization
Multiple (simple) context-free tree grammars are investigated, where "simple"
means "linear and nondeleting". Every multiple context-free tree grammar that
is finitely ambiguous can be lexicalized; i.e., it can be transformed into an
equivalent one (generating the same tree language) in which each rule of the
grammar contains a lexical symbol. Due to this transformation, the rank of the
nonterminals increases at most by 1, and the multiplicity (or fan-out) of the
grammar increases at most by the maximal rank of the lexical symbols; in
particular, the multiplicity does not increase when all lexical symbols have
rank 0. Multiple context-free tree grammars have the same tree generating power
as multi-component tree adjoining grammars (provided the latter can use a
root-marker). Moreover, every multi-component tree adjoining grammar that is
finitely ambiguous can be lexicalized. Multiple context-free tree grammars have
the same string generating power as multiple context-free (string) grammars and
polynomial time parsing algorithms. A tree language can be generated by a
multiple context-free tree grammar if and only if it is the image of a regular
tree language under a deterministic finite-copying macro tree transducer.
Multiple context-free tree grammars can be used as a synchronous translation
device.Comment: 78 pages, 13 figure
Lexicalized non-local MCTAG with dominance links is NP-complete
An NP-hardness proof for non-local Multicomponent Tree Adjoining Grammar
(MCTAG) by Rambow and Satta (1st International Workshop on Tree
Adjoining Grammers 1992), based on Dahlhaus and Warmuth (in J Comput
Syst Sci 33:456–472, 1986), is extended to some linguistically
relevant restrictions of that formalism. It is found that there are
NP-hard grammars among non-local MCTAGs even if any or all of the
following restrictions are imposed: (i) lexicalization: every tree in
the grammar contains a terminal; (ii) dominance links: every tree set
contains at most two trees, and in every such tree set, there is a link
between the foot node of one tree and the root node of the other tree,
indicating that the former node must dominate the latter in the derived
tree. This is the version of MCTAG proposed in Becker et al.
(Proceedings of the 5th conference of the European chapter of the
Association for Computational Linguistics 1991) to account for German
long-distance scrambling. This result restricts the field of possible
candidates for an extension of Tree Adjoining Grammar that would be both
mildly context-sensitive and linguistically adequate
On state-alternating context-free grammars
AbstractState-alternating context-free grammars are introduced, and the language classes obtained from them are compared to the classes of the Chomsky hierarchy as well as to some well-known complexity classes. In particular, state-alternating context-free grammars are compared to alternating context-free grammars (Theoret. Comput. Sci. 67 (1989) 75–85) and to alternating pushdown automata. Further, various derivation strategies are considered, and their influence on the expressive power of (state-) alternating context-free grammars is investigated
Mild context-sensitivity and tuple-based generalizations of context-free grammar
This paper classifies a family of grammar formalisms that extend context-free grammar by talking about tuples of terminal strings, rather than independently combining single terminal words into larger single phrases. These include a number of well-known formalisms, such as head grammar and linear context-free rewriting systems, but also a new formalism, (simple) literal movement grammar, which strictly extends the previously known formalisms, while preserving polynomial time recognizability. The descriptive capacity of simple literal movement grammars is illustrated both formally through a weak generative capacity argument and in a more practical sense by the description of conjunctive cross-serial relative clauses in Dutch. After sketching a complexity result and drawing a number of conclusions from the illustrations, it is then suggested that the notion of mild context-sensitivity currently in use, that depends on the rather loosely defined concept of constant growth, needs a modification to apply sensibly to the illustrated facts; an attempt at such a revision is proposed
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