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arXiv · 1701.02673

A Crevice on the Crane Beach: Finite-Degree Predicates

Michaël Cadilhac·Charles Paperman
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Abstract

First-order logic (FO) over words is shown to be equiexpressive with FO equipped with a restricted set of numerical predicates, namely the order, a binary predicate MSB$_0$, and the finite-degree predicates: FO[Arb] = FO[<, MSB$_0$, Fin]. The Crane Beach Property (CBP), introduced more than a decade ago, is true of a logic if all the expressible languages admitting a neutral letter are regular. Although it is known that FO[Arb] does not have the CBP, it is shown here that the (strong form of the) CBP holds for both FO[<, Fin] and FO[<, MSB$_0$]. Thus FO[<, Fin] exhibits a form of locality and the CBP, and can still express a wide variety of languages, while being one simple predicate away from the expressive power of FO[Arb]. The counting ability of FO[<, Fin] is studied as an application.

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Michaël Cadilhac, Charles Paperman. 2017-01-10. A Crevice on the Crane Beach: Finite-Degree Predicates. https://arxiv.org/abs/1701.02673

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