On the relationship between monadic and weak monadic second order logic on arbitrary trees, with applications to the mu-calculus
Résumé
In1970, Rabin shows that a language is recognizable by a tree automaton with Bu ̈chi 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.
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