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Two-way Automata and Regular Languages of Overlapping Tiles

Dicky A., Janin D.

Fundamenta Informaticae

2015

Vol. 141, nr 4

311--343

We consider classes of languages of overlapping tiles, i.e., subsets of the McAlister monoid: the class REG of languages definable by Kleene’s regular expressions, the class MSO of languages definable by formulas of monadic second-order logic, and the class REC of languages definable by morphisms into finite monoids. By extending the semantics of finite-state two-way automata (possibly with pebbles) from languages of words to languages of tiles, we obtain a complete characterization of the classes REG and MSO. In particular, we show that adding pebbles strictly increases the expressive power of two-way automata recognizing languages of tiles, but the hierarchy induced by the number of allowed pebbles collapses to level one.

On the Relationship Between Monadic and Weak Monadic Second Order Logic on Arbitrary Trees, with Applications to the mu-Calculus

Janin D., Lenzi G.

Fundamenta Informaticae

2004

Vol. 61, nr 3,4

247--265

In 1970, in Weakly definable relations and special automata, Math. Log. and Found. of Set Theory, pp 1-23, Rabin shows that a language is recognizable by a tree automaton with Büchi like infinitary condition if and only if it is definable as the projection of a weakly definable language. In this paper, we refine this result characterizing such languages as those definable in the monadic Σ2 level of the quantifier alternation depth hierarchy of monadic second order logic (MSO). This new result also contributes to a better understanding of the relationship between the quantifier alternation depth of hierarchy of MSO and the fixpoint alternation depth hierarchy of the mu-calculus: it shows that the bisimulation invariant fragment of the monadic Σ2 level equals the νμ-level of the mu-calculus hierarchy.