Context-free languages over infinite alphabets

In this paper we introduce context-free grammars and pushdown automata over infinite alphabets. It is shown that a language is generated by a context-free grammar over an infinite alphabet if and only if it is accepted by a pushdown automaton over an infinite alphabet. Also the generated (accepted) languages possess many of the properties of the ordinary context-free languages: decidability, closure properties, etc.. This provides a substantial evidence for considering context-free grammars and pushdown automata over infinite alphabets as a natural extension of the classical ones. Received November 27, 1995 / March 4, 1997     

格值下推自动机与格值上下文无关文法

引入了格值下推自动机、格值上下文无关文法及它们的语言的概念,证明了格值下推自动机以两种不同方式接受的语言类的等价性,研究了格值Chomsky范式文法、格值上下文无关文法及其派生所产生的语言的等价条件,揭示了在一定条件下,格值下推自动机接受的语言类与格值上下文无关文法产生的语言类的等价性,证明了有理格值语言均被格值下推自动机识别。     

量化上下文无关语言的代数性质

通过引入量化下推自动机与量化上下文无关文法的定义,研究了以两种不同方式接受语言的量化下推自动机等价性问题,证明了在可交换的双幺赋值幺半群上,量化下推自动机接受的语言与量化上下文无关文法生成的语言相同。     

A hierarchy of uniquely parsable grammar classes and deterministic acceptors

We introduce a new class of grammars called uniquely parsable grammars (UPGs). A UPG is a kind of phrase structure grammar having a restricted type of rewriting rules, where parsing can be performed without backtracking. We show that, in spite of such restriction to the rules, UPGs are universal in their generating ability. We then define three subclasses of UPGs. They are M-UPGs (monotonic UPGs), RC-UPGs (UPGs with right-terminating and context-free-like rules), and REG-UPGs (regular UPGs). It is proved that the generating abilities of the classes of M-UPGs, RC-UPGs, and REG-UPGs are exactly characterized by the classes of deterministic linear-bounded automata, deterministic pushdown automata, and deterministic finite automata, respectively. Especially, the class of RC-UPGs gives a very simple grammatical characterization of the class of deterministic context-free languages. Thus, these four classes form a deterministic counterpart of the classical Chomsky hierarchy. Received August 30, 1995 / May 13, 1996     

The Equivalence of Tree Adjoining Grammars and Monadic Linear Context-free Tree Grammars

The equivalence of leaf languages of tree adjoining grammars and monadic linear context-free grammars was shown about a decade ago. This paper presents a proof of the strong equivalence of these grammar formalisms. Non-strict tree adjoining grammars and monadic linear context-free grammars define the same class of tree languages. We also present a logical characterisation of this tree language class showing that a tree language is a member of this class iff it is the two-dimensional yield of an MSO-definable three-dimensional tree language.     

A NEW AUTOMATON MODEL FOR TAGS: 2-SA

In this paper, we introduce and define a new class of automata (pushdown automata with n stacks, abbreviated as n-SA). The ultimate aim is to give a new characterization of LCRFL, the class of languages accepted by a linear context-free rewriting system (LCFRS). In particular, we introduce 2-SA as a new automaton model for tree-adjoining grammars (TAG). In the simplest cases (0-SA and 1-SA), the languages that are accepted by the automata are the regular and context-free languages respectively. A more complex case is the case of a 2-SA which accepts TALs. The n-SA creates an infinite hierarchy of languages and it seems that this hierarchy corresponds to others in the class LCFRL. The 2-SA corresponds closely to the EPDA (embedded pushdown automaton, an automaton model equivalent to TAGs). Unlike the EPDA, which allows push operations "below the top stack," an n-SA allows push and pop operations only on the top of their (multiple) stacks. So n-SA trade simpler operations against an also simpler but expanded storage structure.     

Tree Adjoining Languages and Multipushdown Languages

Many families of grammars have been studied to extend the power of context-free grammars, while retaining the attractive properties of context-free languages. Some of these formalisms have been proved equivalent to tree adjoining grammars ( tag ), introduced by computational linguists to model natural languages. Another family of grammars, multidepth grammars, introduced for describing the syntax of programming languages, generates a hierarchy of languages, called k -pushdown languages (k-pdl ), for k ≥ 1 . Multidepth grammars are simpler than some other formalisms and have a very natural accepting device, called multipushdown automaton. Here we study the relationship of k-pdl with tal , the class of languages generated by tag . We prove that 2-pdl ⊂ tal ⊂ 3-pdl . Moreover, tal can be characterized exactly as the smallest full super-AFL that includes 2-pdl . Received August 1996, and in final form March 15, 2000.     

LINEAR ITERATED PUSHDOWNS

This paper discusses variants of nondeterministic one-way S -automata and context-free S -grammars where S is a storage type. The framework that these systems provide can be used to give alternative formulations of embedded pushdown automata and linear indexed grammars. The embedded pushdown automata is obtained by means of a linear version of a class of storage types called iterated pushdowns. Linear indexed grammar is obtained by using the pushdown storage type and restricting the way in which the grammar uses its storage.     

Pushdown tree automata

We define topdown pushdown tree automata (PDTA's) which extend the usual string pushdown automata by allowing trees instead of strings in both the input and the stack. We prove that PDTA's recognize the class of context-free tree languages. (Quasi)realtime and deterministic PDTA's accept the classes of Greibach and deterministic tree languages, respectively. Finally, PDTA's are shown to be equivalent to restricted PDTA's, whose stack is linear: this both yields a more operational way of recognizing context-free tree languages and connects them with the class of indexed languages.     

Look-ahead on pushdowns

The general notion of look-ahead on pushdowns is used to prove that (1) the deterministic iterated pushdown languages are closed under complementation, (2) the deterministic iterated pushdown languages are properly included in the non-deterministic iterated pushdown languages; the counter example is a very simple linear context-free language, independent of the amount of iteration, (3) LL(k) iterated indexed grammars can be parsed by deterministic iterated pushdown automata, and (4) it is decidable whether an iterated indexed grammar is LL(k). Analogous results hold for iterated pushdown automata with regular look-ahead on the input, and LL-regular iterated indexed grammars.     

Conjunctive grammars and alternating pushdown automata

In this paper we introduce a variant of alternating pushdown automata, synchronized alternating pushdown automata, which accept the same class of languages as those generated by conjunctive grammars.     

An algebraic characterization of some principal regulated rational cones

The aim of this paper is to deal with formal power series over a commutative semiring A. Generalizing Wechler's pushdown automata and pushdown transition matrices yields a characterization of the A-semi-algebraic power series in terms of acceptance by pushdown automata. Principal regulated rational cones generated by cone generators of a certain form are characterized by algebraic systems given in certain matrix form. This yields a characterization of some principal full semi-AFL's in terms of context-free grammars. As an application of the theory, the principal regulated rational cone of one-counter “languages” is considered.     

基于量子逻辑的自动机和文法理论

初步建立了基于量子逻辑的自动机和文法理论的基本框架.引入了量子文法(称为l值文法),特别是证明了任意l值正规文法生成的语言(称为量子语言)等价于某种基于量子逻辑且含动作(的自动机(称为l值自动机)识别的语言,反之,任意l值自动机识别的语言等价于某l值正规文法生成的语言.建立了l值泵引理,并得到量子语言的判定性刻画.最后简要讨论了正规文法与量子文法(即l值正规文法)的关系.因此,为进一步研究更复杂的量子自动机(如量子下推自动机和Turing机)和量子文法(如量子上下文无关文法和上下文有关文法)奠定了基础.     

Tree controlled grammars

Languages are studied which can be generated by context-free grammars under a single simple restriction which must be satisfied by its derivation trees. Using tree controlled grammars (TC grammars for short) all unambigous and some inherently ambigous context-free languages, and also some non context-free languages can be parsed in timeO(n ²). The classes of regular, linear, context-free, EOL, ETOL and type 0 languages can be characterized in a natural manner using TC grammars. A context-free generator for all type 0 languages is exhibited. Some normal forms for TC grammars are established but it is shown that many common normal forms (e. g. Greibach normal form) cannot be obtained for TC grammars in general.     

Linearity and nondeletion on monadic context-free tree grammars

In this paper, subclasses of monadic context-free tree grammars (CFTGs) are compared. Since linear, nondeleting, monadic CFTGs generate the same class of string languages as tree adjoining grammars (TAGs), it is examined whether the restrictions of linearity and nondeletion on monadic CFTGs are necessary to generate the same class of languages.     

On regular tree languages and deterministic pushdown automata

The theory of formal string languages and of formal tree languages are both important parts of the theory of formal languages. Regular tree languages are recognized by finite tree automata. Trees in their postfix notation can be seen as strings. This paper presents a simple transformation from any given (bottom-up) finite tree automaton recognizing a regular tree language to a deterministic pushdown automaton accepting the same tree language in postfix notation. The resulting deterministic pushdown automaton can be implemented easily by an existing parser generator because it is constructed for an LR(0) grammar, and its size directly corresponds to the size of the deterministic finite tree automaton. The class of regular tree languages in postfix notation is a proper subclass of deterministic context-free string languages. Moreover, the class of tree languages which are in their postfix notation deterministic context-free string languages is a proper superclass of the class of regular tree languages.     

The equivalence of four extensions of context-free grammars

There is currently considerable interest among computational linguists in grammatical formalisms with highly restricted generative power. This paper concerns the relationship between the class of string languages generated by several such formalisms, namely, combinatory categorial grammars, head grammars, linear indexed grammars, and tree adjoining grammars. Each of these formalisms is known to generate a larger class of languages than context-free grammars. The four formalisms under consideration were developed independently and appear superficially to be quite different from one another. The result presented in this paper is that all four of the formalisms under consideration generate exactly the same class of string languages.This work has been supported by NSF Grants MCS-82-19116-CER, MCS-82-07294, DCR-84-10413, IRI-8909810, ARO Grant DAA29-84-9-0027, and DARPA Grant N0014-85-K0018.     

格值树自动机与格值上下文无关树文法的等价性

本文将模糊树自动机和模糊上下文无关树文法的概念推广到格半群上。证明了在接受语言和生成语言的意义下,树自动机和上下文无关树文法是等价的。同时给出了构造正规形式的等价文法的方法。     

Formal Languages: Origins and Directions

Origins of the theory of formal languages and automata are surveyed starting from 1936 with the work of Turing and Post. Special attention is given to the machine translation projects of the 1950s and early 1960s and associated work in mathematical linguistics. The development of the Chomsky hierarchy of grammars, machines, and languages from 1956 to 1964 is traced. It is observed that the same important ideas emerged independently for the automatic analysis and translation of both natural and artificial languages. Since 1964, formal language theory is part of theoretical computer science. A few of the directions since 1964 are considered: restrictions and extensions of context-free grammars and pushdown store automata, unifying frameworks, and complexity questions.     

A mechanisation of some context-free language theory in HOL4

We describe the mechanisation of some foundational results in the theory of context-free languages (CFLs), using the HOL4 system. We focus on pushdown automata (PDAs). We show that two standard acceptance criteria for PDAs (“accept-by-empty-stack” and “accept-by-final-state”) are equivalent in power. We are then able to show that the pushdown automata (PDAs) and context-free grammars (CFGs) accept the same languages by showing that each can emulate the other. With both of these models to hand, we can then show a number of basic, but important results. For example, we prove the basic closure properties of the context-free languages such as union and concatenation. Along the way, we also discuss the varying extent to which textbook proofs (we follow Hopcroft and Ullman) and our mechanisations diverge: sometimes elegant textbook proofs remain elegant in HOL; sometimes the required mechanisation effort blows up unconscionably.