Marcella Anselmo

Unambiguous recognizable two-dimensional languages

We consider the family UREC of unambiguous recognizable two-dimensional languages. We prove that there are recognizable languages that are inherently ambiguous, that is UREC family is a proper subclass of REC family. The result is obtained by showing a necessary condition for unambiguous recognizable languages. Further UREC family coincides with the class of picture languages defined by unambiguous 2OTA and it strictly contains its deterministic counterpart. Some closure and non-closure properties of UREC are presented. Finally we show that it is undecidable whether a given tiling system is unambiguous.

Deterministic and Unambiguous Families within Recognizable Two-dimensional Languages

Recognizable two-dimensional languages (REC) are defined by tiling systems that generalize to two dimensions non-deterministic finite automata for strings. We introduce the notion of deterministic tiling system and the corresponding family of languages (DREC) and study its structural and closure properties. Furthermore we show that, in contrast with the one-dimensional case, there exist other classes between deterministic and non-deterministic families that we separate by means of examples and decidability properties.

Deterministic and unambiguous two-dimensional languages over one-letter alphabet

The paper focuses on deterministic and unambiguous recognizable two-dimensional languages with particular attention to the case of a one-letter alphabet. The family DREC(1) of deterministic languages over a one-letter alphabet is characterized as both L(DOTA)(1), the class of languages accepted by deterministic on-line tessellation acceptors, and L(2AFA)(1), the class of languages recognized by 2-way alternating finite automata. We show that there are inherently ambiguous languages and unambiguously recognizable languages that cannot be deterministically recognized even in the case of a one-letter alphabet. In particular we show that on-line tessellation acceptors are more powerful than their deterministic counterpart, even in the case of a one-letter alphabet. Finally we show that DREC(1) is complex enough not to be characterized in terms of classical operations.

A computational model for tiling recognizable two-dimensional languages

Tiling systems are a well accepted model to define recognizable two-dimensional languages but they are not an effective device for recognition unless a scanning strategy for the pictures is fixed. We define a tiling automaton as a tiling system equipped with a scanning strategy and a suitable data structure. The class of languages accepted by tiling automata coincides with the REC family. In this framework it is possible to define determinism, non-determinism and unambiguity. Then (deterministic) tiling automata are compared with the other known (deterministic) automata models for two-dimensional languages.

New operations and regular expressions for two-dimensional languages over one-letter alphabet

We consider the problem of defining regular expressions to characterize the class of recognizable picture languages in the case of a one-letter alphabet. We define a diagonal concatenation and its star and consider two different families, L(D) and L(CRD), of languages denoted by regular expressions involving such operations plus classical operations. L(D) is characterized both in terms of rational relations and in terms of two-dimensional automata moving only right and down. L(CRD) is included in REC and contains languages defined by three-way automata while languages in L(CRD) necessarily satisfy some regularity conditions. Finally, we introduce new definitions of advanced stars expressing the necessity of conceptually different definitions for iteration.

ON LANGUAGES FACTORIZING THE FREE MONOID

A language X⊂A* is called factorizing if there exists a language Y⊂A* such that XY = A* This work was partially supported by ESPRIT-EBRA project ASMICS contact 6317 and project 40% MURST “Algoritmi, Modelli di Calcolo e Strutture Informative”. and the product is unambiguous. First we give a combinatorial characterization of factorizing languages. Further we prove that it is decidable whether a regular language X is factorizing and we construct an automaton recognizing the corresponding language Y. For finite languages we show that it suffices to consider words of bounded length. A complete characterization of factorizing languages with three words and explicit regular expression for the corresponding language Y are also given. Finally we prove a more general result stating that, given two regular languages X and T, it is decidable whether there exists a language Y such that XY=T and the product is unambiguous.

Two-way automata with multiplicities

We introduce the notion of two-way automata with multiplicity in a semiring. Our main result is the extension of Rabin, Scott and Shepherdsons Theorem to this more general case. We in fact show that it holds in the case of automata with multiplicity in a commutative semiring, provided that an additional condition is satisfied. We prove that this condition is also necessary in a particular case. An application is given to zig-zag codes using special two-way automata.

Finite automata and non-self-embedding grammars

We consider non-self-embedding (NSE) context-free grammars as a representation of regular sets.We point out its advantages with respect to more classical representations by finite automata, in particular when considering the efficient realization of the rational operations. We give a characterization in terms of composition of regular grammars and state relationships between NSE grammars and push-down automata. Finally we show a polynomial algorithm to decide whether a context-free grammars is self-embedding or not.

Tiling automaton: a computational model for recognizable two-dimensional languages

Two-dimensional languages can be recognized by tiling systems. A tiling system becomes an effective device for recognition when a scanning strategy on pictures is fixed. We define a Tiling Automaton as a tiling system together with a scanning strategy and a suitable data structure. In this framework it is possible to define determinism, nondeterminism and unambiguity. The class of languages accepted by tiling automata coincides with REC family. Tiling automata are able to simulate on-line tessellation automata. Then (deterministic) tiling automata are compared with the other known (deterministic) automata models for recognition of two-dimensional languages.

Prefix picture codes: A decidable class of two-dimensional codes

A two-dimensional code of pictures is defined as a set X ⊆ Σ** such that any picture over Σ is tilable in at most one way with pictures in X. It is proved that in general it is undecidable whether a finite set of picture is a code. The subclass of prefix codes is introduced and it is proved that it is decidable whether a finite set of pictures is a prefix code. Further a polynomial time decoding algorithm for finite prefix codes is given. Maximality and completeness of finite prefix codes are studied.