A New Algorithm for Normal Dominance Constraints Manuel Bodirsky Denys Duchier† Joachim Niehren‡ Sebastian Miele§

Manuel Bodirsky

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Niveau: Supérieur, Doctorat, Bac+8
A New Algorithm for Normal Dominance Constraints Manuel Bodirsky? Denys Duchier† Joachim Niehren‡ Sebastian Miele November 12, 2003 Dominance constraints are logical descriptions of trees. Efficient algorithms for the subclass of normal dominance constraints were recently proposed. We present a new and simpler graph algorithm solving these constraints more efficiently, in quadratic time per solved form. It also applies to weakly normal dominance constraints as needed for an application to computational linguistics. Subquadratic running time can be achieved employing decremental graph biconnectivity algorithms. 1 Introduction Dominance constraints are logical descriptions of trees [2, 10] that can talk about the mother and ancestor re- lations between the nodes of a tree. They have numer- ous applications, most of which belong to the area of computational linguistics, e.g. in underspecified seman- tics [4–6], underspecified discourse [7], and parsing with tree adjoining grammar [12]. Satisfiability of dominance constraints was proved NP-complete in [9]. This shed doubts on the feasibility of dominance based applications. The doubts were removed by Mehlhorn et al [1], who distinguished the language of normal dominance constraints which suffices for many applications and has a polynomial time satisfiability problem. The most relevant problem for normal dominance constraints is to enumerate solved forms, i.e., all trees satisfying a constraint. Mehlhorn et al [1] presented an enumeration algorithm whose running time is O(n4) per solved form.

• solved forms


• weakly normal


• dominance constraint


• all solved


• can choose


• dominance graph

Informations

• Publié par: profil-zyak-2012
• Nombre de lectures: 10
• Langue: English

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A New Algorithm for Normal Dominance Constraints

∗ Manuel Bodirsky

† Denys Duchier

‡ Joachim Niehren

November12,2003

Dominance constraints are logical descriptions of trees. Efficient algorithms for the subclass ofnormal dominance constraintswere recently proposed. We present a new and simpler graph algorithm solving these constraints more efficiently, in quadratic time per solved form. It also applies to weakly normal dominance constraints as needed for an application to computational linguistics. Subquadratic running time can be achieved employing decremental graph biconnectivity algorithms.

1 Introduction Dominance constraints are logical descriptions of trees [2, 10] that can talk about the mother and ancestor re- lations between the nodes of a tree. They have numer- ous applications, most of which belong to the area of computational linguistics, e.g. in underspecified seman- tics [4–6], underspecified discourse [7], and parsing with tree adjoining grammar [12]. Satisfiability of dominance constraints was proved NP-complete in [9]. This shed doubts on the feasibility of dominance based applications. The doubts were removed by Mehlhorn et al [1], who distinguished the language ofnormal dominance constraintswhich suffices for many applications and has a polynomial time satisfiability problem. The most relevant problem for normal dominance constraints is to enumerate solved forms, i.e., all trees satisfying a constraint. Mehlhorn et al [1] presented 4 an enumeration algorithm whose running time isO(n) per solved form. This algorithm relies on an efficient satisfiability test. Thiel [13] improved this result to 3 O(n) by faster satisfiability testing. In this paper, we propose a novel graph algorithm relying on graph connectivity, and inspired by [3]. It can enumerate all solved forms of a normal dominance con- 2 straint inO(n) per solved form, and thereby improves on the best previously known algorithm in efficiency.

∗ HumboldtUniversit¨atzuBerlin,Germany † LORIA, Nancy, France ‡ INRIAteamMostrare,Universit´edeLille,France § Universit¨atdesSaarlandes,Germany

§ Sebastian Miele

Subquadratic running time can be achieved employing decremental graph biconnectivity algorithms. Our algorithm applies to the extended language ofweakly normal dominance constraintsthat we intro- duce. This improves the applicability of dominance con- straints in the area of natural language semantics. It un- derlies the first polynomial time algorithm forminimal recursion semantics (MRS)[5] presented in a follow up paper [11]. MRS is built intohead driven phrase struc- ture grammar (HPSG), one of the mainstream grammar formalisms in computational linguistics.

2 Weakly Normal Dominance Constraints We start with a brief exposition of dominance con- straints [2, 10], recall the notion of normality [1], and then introduce weak normality. Letf, grange over the elements of some signature of function symbols with fixed arities anda, bover con- stants, i.e., function symbols of arity 0. Aconstructor treeover this signature is a ground termτconstructed from the function symbols, as for instancef(g(a, a)). We identify ground terms with trees that are rooted, ranked, edge-ordered, and node-labeled. See Fig. 1 for a graphical representation of the treef(g(a, a)).

a

f g

a

Figure 1:f(g(a, a))

Dominance constraints are logic formulas that de- scribe constructor trees. They can talk about the mother-child relationbetween the nodes of a tree, ∗ dominancewhich is the reflexive ancestor relation, and inequality6=. LetX, Y, Zrange over an infinite set ofnode variables. Adominance constraintφis then a conjunction of literals:

φ

::=

∗ 0 00 X:f(Y 1 , . . . , Y n )|X Y|X6=Y|φ∧φ

A solution of a dominance constraint consists of a tree τand a variable assignmentαto the nodes ofτ. A