scholarly journals On the Density of Context-Free and Counter Languages

2018 ◽  
Vol 29 (02) ◽  
pp. 233-250 ◽  
Author(s):  
Joey Eremondi ◽  
Oscar H. Ibarra ◽  
Ian McQuillan
Keyword(s):  
Pushdown Automaton ◽  
Standard Notion ◽  
Context Free ◽  
The Universe ◽  
Pushdown Automata

A language [Formula: see text] is said to be dense if every word in the universe is an infix of some word in [Formula: see text]. This notion has been generalized from the infix operation to arbitrary word operations [Formula: see text] in place of the infix operation ([Formula: see text]-dense, with infix-dense being the standard notion of dense). It is shown here that it is decidable, for a language [Formula: see text] accepted by a one-way nondeterministic reversal-bounded pushdown automaton, whether [Formula: see text] is infix-dense. However, it becomes undecidable for both deterministic pushdown automata (with no reversal-bound), and for nondeterministic one-counter automata. When examining suffix-density, it is undecidable for more restricted families such as deterministic one-counter automata that make three reversals on the counter, but it is decidable with less reversals. Other decidability results are also presented on dense languages, and contrasted with a marked version called [Formula: see text]-marked-density. Also, new languages are demonstrated to be outside various deterministic language families after applying different deletion operations from smaller families. Lastly, bounded-dense languages are defined and examined.

EFFICIENT DETECTORS AND CONSTRUCTORS FOR SIMPLE LANGUAGES

1991 ◽  
Vol 02 (03) ◽  
pp. 183-205 ◽  
Author(s):  
Dung T. Huynh
Keyword(s):  
Finite Automata ◽  
Boolean Circuits ◽  
Turing Machines ◽  
Constant Depth ◽  
Pushdown Automaton ◽  
Polynomial Size ◽  
Regular Sets ◽  
Context Free ◽  
Pushdown Automata ◽  
Nondeterministic Logspace

In this paper, we investigate the complexity of computing the detector, constructor and lexicographic constructor functions for a given language. The following classes of languages will be considered: (1) context-free languages, (2) regular sets, (3) languages accepted by one-way nondeterministic auxiliary pushdown automata, (4) languages accepted by one-way nondeterministic logspace-bounded Turing machines, (5) two-way deterministic pushdown automaton languages, (6) languages accepted by uniform families of constant-depth polynomial-size Boolean circuits, and (7) languages accepted by multihead finite automata. We show that for the classes (1)–(4), efficient detectors, constructors and lexicographic constructors exist, whereas for (5)– (7) polynomial-time computable detectors, constructors and lexicographic constructors exist iff there are no sparse sets in NP−P (or equivalently, E=NE). Our results provide sharp boundaries between classes of languages which have efficient detectors, constructors and classes of languages for which efficient detectors and constructors do not appear to exist.

scholarly journals DETERMINISTIC PUSHDOWN AUTOMATA AND UNARY LANGUAGES

2009 ◽  
Vol 20 (04) ◽  
pp. 629-645 ◽  
Author(s):  
GIOVANNI PIGHIZZINI
Keyword(s):  
Finite Automata ◽  
Point Of View ◽  
Deterministic Finite Automaton ◽  
Pushdown Automaton ◽  
Nondeterministic Automaton ◽  
Letter Alphabet ◽  
Finite State ◽  
Unary Languages ◽  
Context Free ◽  
Pushdown Automata

The simulation of deterministic pushdown automata defined over a one-letter alphabet by finite state automata is investigated from a descriptional complexity point of view. We show that each unary deterministic pushdown automaton of size s can be simulated by a deterministic finite automaton with a number of states that is exponential in s. We prove that this simulation is tight. Furthermore, its cost cannot be reduced even if it is performed by a two-way nondeterministic automaton. We also prove that there are unary languages for which deterministic pushdown automata cannot be exponentially more succinct than finite automata. In order to state this result, we investigate the conversion of deterministic pushdown automata into context-free grammars. We prove that in the unary case the number of variables in the resulting grammar is strictly smaller than the number of variables needed in the case of nonunary alphabets.

scholarly journals Subtree matching by pushdown automata

2010 ◽  
Vol 7 (2) ◽  
pp. 331-357 ◽  
Author(s):  
Tomás Flouri ◽  
Jan Janousek ◽  
Bořivoj Melichar
Keyword(s):  
Programming Languages ◽  
Theorem Proving ◽  
Systematic Approach ◽  
Finite Automata ◽  
Term Rewriting ◽  
Mechanical Theorem Proving ◽  
Pushdown Automaton ◽  
String Pattern ◽  
Pushdown Automata ◽  
Mechanical Theorem

Subtree matching is an important problem in Computer Science on which a number of tasks, such as mechanical theorem proving, term-rewriting, symbolic computation and nonprocedural programming languages are based on. A systematic approach to the construction of subtree pattern matchers by deterministic pushdown automata, which read subject trees in prefix and postfix notation, is presented. The method is analogous to the construction of string pattern matchers: for a given pattern, a nondeterministic pushdown automaton is created and is then determinised. In addition, it is shown that the size of the resulting deterministic pushdown automata directly corresponds to the size of the existing string pattern matchers based on finite automata.

scholarly journals Pushdown Automata in Statistical Machine Translation

2014 ◽  
Vol 40 (3) ◽  
pp. 687-723 ◽  
Author(s):  
Cyril Allauzen ◽  
Bill Byrne ◽  
Adrià de Gispert ◽  
Gonzalo Iglesias ◽  
Michael Riley
Keyword(s):  
Machine Translation ◽  
Large Scale ◽  
Complexity Analysis ◽  
Statistical Machine Translation ◽  
Language Model ◽  
General Purpose ◽  
Language Models ◽  
Experimental Conditions ◽  
Context Free ◽  
Pushdown Automata

This article describes the use of pushdown automata (PDA) in the context of statistical machine translation and alignment under a synchronous context-free grammar. We use PDAs to compactly represent the space of candidate translations generated by the grammar when applied to an input sentence. General-purpose PDA algorithms for replacement, composition, shortest path, and expansion are presented. We describe HiPDT, a hierarchical phrase-based decoder using the PDA representation and these algorithms. We contrast the complexity of this decoder with a decoder based on a finite state automata representation, showing that PDAs provide a more suitable framework to achieve exact decoding for larger synchronous context-free grammars and smaller language models. We assess this experimentally on a large-scale Chinese-to-English alignment and translation task. In translation, we propose a two-pass decoding strategy involving a weaker language model in the first-pass to address the results of PDA complexity analysis. We study in depth the experimental conditions and tradeoffs in which HiPDT can achieve state-of-the-art performance for large-scale SMT.

scholarly journals THE CHOMSKY-SCHÜTZENBERGER THEOREM FOR QUANTITATIVE CONTEXT-FREE LANGUAGES

2014 ◽  
Vol 25 (08) ◽  
pp. 955-969 ◽  
Author(s):  
MANFRED DROSTE ◽  
HEIKO VOGLER
Keyword(s):  
Algebraic Structure ◽  
Weighted Automata ◽  
General Weight ◽  
Context Free ◽  
Pushdown Automata ◽  
Recognizable Language ◽  
Weight Structures

Weighted automata model quantitative aspects of systems like the consumption of resources during executions. Traditionally, the weights are assumed to form the algebraic structure of a semiring, but recently also other weight computations like average have been considered. Here, we investigate quantitative context-free languages over very general weight structures incorporating all semirings, average computations, lattices. In our main result, we derive the Chomsky-Schützenberger Theorem for such quantitative context-free languages, showing that each arises as the image of the intersection of a Dyck language and a recognizable language under a suitable morphism. Moreover, we show that quantitative context-free languages are expressively equivalent to a model of weighted pushdown automata. This generalizes results previously known only for semirings. We also investigate under which conditions quantitative context-free languages assume only finitely many values.

CLOSURE PROPERTY OF SPACE-BOUNDED TWO-DIMENSIONAL ALTERNATING TURING MACHINES, PUSHDOWN AUTOMATA, AND COUNTER AUTOMATA

2001 ◽  
Vol 15 (07) ◽  
pp. 1143-1165 ◽  
Author(s):  
TOKIO OKAZAKI ◽  
KATSUSHI INOUE ◽  
AKIRA ITO ◽  
YUE WANG
Keyword(s):  
Turing Machine ◽  
Nondecreasing Function ◽  
Closure Property ◽  
Two Dimensional ◽  
Turing Machines ◽  
Pushdown Automaton ◽  
Alternating Turing Machines ◽  
Positive Integers ◽  
Pushdown Automata

This paper investigates closure property of the classes of sets accepted by space-bounded two-dimensional alternating Turing machines (2-atm's) and space-bounded two-dimensional alternating pushdown automata (2-apda's), and space-bounded two-dimensional alternating counter automata (2-aca's). Let L(m, n): N2 → N (N denotes the set of all positive integers) be a function with two variables m (= the number of rows of input tapes) and n (= the number of columns of input tapes). We show that (i) for any function f(m) = o( log m) (resp. f(m) = o( log m/ log log m)) and any monotonic nondecreasing function g(n) space-constructible by a two-dimensional Turing machine (2-Tm) (resp. two-dimensional pushdown automaton (2-pda)), the class of sets accepted by L(m,n) space-bounded 2-atm's (2-apda's) is not closed under row catenation, row + or projection, and (ii) for any function f(m) = o(m/ log ) (resp. for any function f(m) such that log f(m) = o( log m)) and any monotonic nondecreasing function g(n) space-constructible by a two-dimensional counter automaton (2-ca), the class of sets accepted by L(m, n) space-bounded 2-aca's is not closed under row catenation, row + or projection, where L(m, n) = f(m) + g(n) (resp. L(m, n) = f(m) × g(n)).

Investigations on Automata and Languages Over a Unary Alphabet

2015 ◽  
Vol 26 (07) ◽  
pp. 827-850 ◽  
Author(s):  
Giovanni Pighizzini
Keyword(s):  
Computational Models ◽  
The State ◽  
Space Complexity ◽  
Finite State Automata ◽  
Probabilistic Automata ◽  
Letter Alphabet ◽  
Finite State ◽  
Context Free ◽  
Pushdown Automata

The investigation of automata and languages defined over a one letter alphabet shows interesting differences with respect to the case of alphabets with at least two letters. Probably, the oldest example emphasizing one of these differences is the collapse of the classes of regular and context-free languages in the unary case (Ginsburg and Rice, 1962). Many differences have been proved concerning the state costs of the simulations between different variants of unary finite state automata (Chrobak, 1986, Mereghetti and Pighizzini, 2001). We present an overview of these results. Because important connections with fundamental questions in space complexity, we give emphasis to unary two-way automata. Furthermore, we discuss unary versions of other computational models, as probabilistic automata, one-way and two-way pushdown automata, even extended with auxiliary workspace, and multi-head automata.

State Complexity of the Quotient Operation on Input-Driven Pushdown Automata

2019 ◽  
Vol 30 (06n07) ◽  
pp. 1217-1235
Author(s):  
Alexander Okhotin ◽  
Kai Salomaa
Keyword(s):  
Regular Language ◽  
Formal Language ◽  
State Complexity ◽  
The Family ◽  
Visibly Pushdown Automata ◽  
Context Free ◽  
Pushdown Automata

The quotient of a formal language [Formula: see text] by another language [Formula: see text] is the set of all strings obtained by taking a string from [Formula: see text] that ends with a suffix of a string from [Formula: see text], and removing that suffix. The quotient of a regular language by any language is always regular, whereas the context-free languages and many of their subfamilies, such as the linear and the deterministic languages, are not closed under the quotient operation. This paper establishes the closure of the family of languages recognized by input-driven pushdown automata (IDPDA), also known as visibly pushdown automata, under the quotient operation. A construction of automata representing the result of the operation is given, and its state complexity with respect to nondeterministic IDPDA is shown to be exactly [Formula: see text], where [Formula: see text] and [Formula: see text] are the numbers of states in the automata recognizing [Formula: see text] and [Formula: see text], respectively.

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