Multi-Structural Games and Number of Quantifiers

04/30/2021

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by   Ronald Fagin, et al.

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We study multi-structural games, played on two sets 𝒜 and ℬ of structures. These games generalize Ehrenfeucht-Fraïssé games. Whereas Ehrenfeucht-Fraïssé games capture the quantifier rank of a first-order sentence, multi-structural games capture the number of quantifiers, in the sense that Spoiler wins the r-round game if and only if there is a first-order sentence ϕ with at most r quantifiers, where every structure in 𝒜 satisfies ϕ and no structure in ℬ satisfies ϕ. We use these games to give a complete characterization of the number of quantifiers required to distinguish linear orders of different sizes, and develop machinery for analyzing structures beyond linear orders.