Abstract
Buchi inLecture Notes in Mathematics, Decidable Theories II (1973) by using A.C. characterized the theoriesMT[β, <] forβ<ω 1 and showed thatMT[ω 1, <] is decidable. We extend Buchi’s results to a larger class of models of ZF (without A.C.) by proving the following under ZF only: (1) There is a choice function which chooses a “good” run of an automaton on countable input (Lemma 5.1). It follows that Buchi’s results cocerning countable ordinals are provable within ZF. (2) Let U.D. be the assertion that there exists a uniform denumeration ofω 1 (i.e. a functionf: ω 1 → ω ω1 such that for everyα<ω 1,f(α) is a function fromω ontoα). We show that U.D. can be stated as a monadic sentence, and thereforeω 1 is characterizable by a sentence. (3) LetF be the filter of the cofinal closed subsets ofω 1. We show that if U.D. holds thenMT[ω 1, <] is recursive in the first order theory of the boolean algebraP (ω 1)/F. (We can effectively translate each monadic sentence Σ to a boolean sentenceσ such that [ω 1, <] ⊨ Σ iffP(ω 1)/F⊨σ). (4) As every complete boolean algebra theory is recursive we have that in every model of ZF+U.D.,MT[ω 1, <] is recursive. All our proofs are within ZF. Buchi’s work is often referred to. Following Buchi, the main tool is finite automata. We don’t deal withMT[ω 1, <] forω 1 which doesn’t satisfy U.D.
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References
J. R. Buchi,The monadic second order theory of ω t, inLecture Notes in Mathematics, Decidable Theories II, Springer-Verlag, Berlin, Heidelberg, New York, 1973, p. 328.
J. R. Buchi and D. Siefkes,Axiomatization of the monadic second order theory of ω 1, inLecture Notes in Mathematics, Decidable Theories II, Springer-Verlag, Berlin, Heidelberg, New York, 1973.
C. C. Chang and H. J. Keisler,Model Theory, North-Holland, Amsterdam, 1973.
R. McNaughton,Testing and generating infinite sequences by a finite automaton, Information and Control9 (1966).
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The results in this paper appeared in the author’s M.Sc. thesis, which was prepared at the Hebrew University under the supervision of Professor M. Rabin.
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Litman, A. On the monadic theory ofω 1 without A.C.. Israel J. Math. 23, 251–266 (1976). https://doi.org/10.1007/BF02761803
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DOI: https://doi.org/10.1007/BF02761803