9307461
Context Free Normal Systems and ETOL Systems
Abstract The relationship between extended context free normal systems (a nondeterministic version of Tag systems of Post with deletion number equal to 1 and using nonterminals) and ETOL systems is considered. It is demonstrated that the class of languages generated by context free normal systems (denoted L (ECFN)) lies strictly between the clas of EOL languages (denoted L (EOL)) and the class of ETOL languages. Several characterizations of L (ECFN)) in terms of L (EOL)) are provided and a number of closure properties of L '(ECFN) are established.
17444444
The Bounded Degree Problem for NLC Grammars is Decidable
Abstract The degree of a graph H is the maximum among the degrees of its nodes. A set of graphs L is of bounded degree if there exists a positive integer n such that the degree of each graph in L does not exceed n . We demonstrate that it is decidable whether or not the (graph) language of an arbitrary node label controlled (NLC) grammar is of bounded degree. Moreover, it is shown that, given an arbitrary NLC grammar G generating the language L(G) of bounded degree, one can effectively compute the maximum integer which appears as the degree of a graph in L(G) .
21626854
An Observation on Scattered Grammars
Scattered context-grammars were introduced in [ 1 j. They constitute one of the most interesting examples of string rewriting systems that are ‘context-dependent’. However they form one of the least understood types of grammars. Although the relationship of (some of the varier.-.._ _ -+i-~ of! the class of scattered context grammars to some classes of languages was studied (see e.g. [2,31) the main question remains open: Do scattered context grammars generate all context sensitive languages’! Clearly an answer to this question would shed light on the role that context plays in grammars. In this work we show that recursively enumerable languages possess a very strong representation in the class of scattered context languages. As a matter of fact it is well known th!at they possess the same representation in the class of context sensitive languages! Thus a result of this form supports in some sense the conjecture that the class of scattered context languages equals the class of context sensitive languages. In any case it says that the class of scattered context languages is ‘close to’ the class of context sensitive languages. Since we provide a direct proof of !2!r reslult (an explicit construction is given) it gives some insight iI& how scattered grammars work. We assume the reader to be familiar with Post Ilormal systems (e.g. in the scope of [4j). If H is a Post normal system, c4 is its axiom and al$ --) S&, ar*S --, S& are productions of l-l, then wr zlrssume that loill = ia21 SK Iwl and Ipli, l&l >O. The language of H consists of al! the words over its alphabet for which
43629107
On the Separating Power of Eol Systems
38741370
Repetitions in Homomorphisms and Languages
39400225
On Subword Complexities of Homomorphic Images of Languages
— The subword complexity of a language K is the function nK on positive integer s such that nK (n) equals the number of different subwords of length n appearing in words of X. We investigate the relationship between nk(K) and nK where Kis a language and h is a homorphism. This study is aiso carried out for the special case when Kis a DOL language. Resumé. — La complexité des facteurs d'un langage K est la fonction nK définie sur les entiers positifs telle que nK (n) est le nombre de facteurs distincts de longueur n apparaissant dans les mots de K. Nous étudions les relations existant entre KH(K) et nKi où K est un langage et h est un homomorphisme. Cette étude est menée également dans le cas particulier où Kest un DOL langage.
38594002
Using String Languages to Describe Picture Languages
A picture is a set of unit lines from the Cartesian plane considered as a square grid. A word over the alphabet l, r, u, d is a picture description in the sense that it represents a traversal of a picture where the interpretation of the symbols l, r, u, d, is: l go one unit line to the left of the current point r go one unit line to the right of the current point u go one unit line up from the current point, and d go one unit line down from the current point. A set of picture descriptions forms a picture description language . This paper investigates the basic properties of pictures and picture description languages from the formal language theory point of view.
37152120
Transition Systems, Event Structures and Unfoldings
A subclass of transition systems called elementary transition systems can be identified with the help of axioms based on a structural notion called regions. Elementary transition systems have been shown to be the transition system model of a basic system model of net theory called elementary net systems. Here we show that by smoothly strengthening the regional axioms for elementary transition systems, one obtains a subclass called occurrence transition system. We then prove that occurrence transition systems are the transition system model of yet another basic model of concurrency, namely, prime event structures. We then propose an operation of unfolding elementary transition systems into occurrence transition systems, We prove that it is "correct" in a strong categorical sense.
28657554
Hypergraph Systems and Their Extensions
The notion of a graph is a natural generalization of the notion of a string. Using one graph (the transition graph ofafinite automaton) one can define a language {the language of a given automaton). This well-known idea is generalized as follows: the notion of a hypergraph is a natural generalization of the notion of a graph. Using one hypergraph (equipped with an additional graph structure) one can define a graph language (a set ofgraphs). Several variants of graph grammars based on this idea are introduced in this paper together withformalism needed to investigate them and illustrating examples. The generating power of various graph-language-generating Systems is compared. Résumé. — La notion de graphe est une généralisation naturelle de la notion de mot. En utilisant un graphe {le graphe de transition d'un automate fini) on définit un langage {le langage accepté par l'automate). Cette idée est généralisée comme suit ; la notion de hypergraphe est une généralisation naturelle de la notion de graphe. Avec un hypergraphe (muni d'une structure de graphe supplémentaire) on peut définir un langage (un ensemble) de graphes. Dans cet article on introduit diverses variantes des grammaires de graphes basées sur cette idée; on donne le formalisme nécessaire pour les étudier, et on les illustre par des exemples. La puissance générative de divers systèmes engendrant des langages de graphes est comparée.
6414809
Developmental systems and languages
Developmental systems were introduced (Lindenmayer, 1968, 1971) in order to model morphogenetic (pattern-generating) processes in growing, multicellular, filamentous organisms. These systems were originally conceived as linear arrays of interconnected finite automata, each automaton corresponding to a living cell, with the possibility that new automata can be added to the array (cells divide) or be deleted from the array (cells die).
 Each cell in the array is supposed to have the same state-transition and output functions. As required by biological considerations these functions must be applied to all cells in the array simultaneously at each time step. Thus one obtains infinite sequences of arrays once the functions and the initial arrays are specified. Simplified constructs are defined (and used in this paper) by considering the states and outputs to be identical and thus omitting the output functions. Such filamentous developmental systems have been called “Lindenmayer models” Herman, 1969, 1970) or “L-systems” (Van Dalen, 1971).
34528489
Reductions for Primitive 2-Structures
A subset X of a 2-structure (a reversible edge-colored directed graph) g is a clan, if X cannot be distinguished by colors from outside of X. We show that if g is primitive, i.e. it has no nontrivial clans, then there exists an edge e or an end vertex x such that when e or x is removed from g, the resulting 2-structure g′ remains primitive. We also present a composition result for the unstable 2-structures, which are those primitive 2-structures where a removal of any edge results in a nonprimitive 2-structure.