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  1. Boffa’s construction and models for NFU.Tin Adlešić & Vedran Čačić - forthcoming - Studia Logica:1-25.
    New Foundations with Urelements (NFU) is a theory that extends Quine’s original theory (New Foundations) by adding “urelements” (atoms). It was discovered by Jensen in 1969, who proved that NFU is relatively consistent with Peano arithmetic and consequently with Zermelo–Fraenkel set theory (ZF). Jensen’s proof is rather hard to follow, so Boffa introduced a more straightforward method of constructing models for NFU from a model of ZF. However, Boffa’s presentation of his construction is extremely terse with many essential details omitted, (...)
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  • On the relative strengths of fragments of collection.Zachiri McKenzie - 2019 - Mathematical Logic Quarterly 65 (1):80-94.
    Let be the basic set theory that consists of the axioms of extensionality, emptyset, pair, union, powerset, infinity, transitive containment, Δ0‐separation and set foundation. This paper studies the relative strength of set theories obtained by adding fragments of the set‐theoretic collection scheme to. We focus on two common parameterisations of the collection: ‐collection, which is the usual collection scheme restricted to ‐formulae, and strong ‐collection, which is equivalent to ‐collection plus ‐separation. The main result of this paper shows that for (...)
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  • On the strength of a weak variant of the axiom of counting.Zachiri McKenzie - 2017 - Mathematical Logic Quarterly 63 (1-2):94-103.
    In this paper is used to denote Jensen's modification of Quine's ‘new foundations’ set theory () fortified with a type‐level pairing function but without the axiom of choice. The axiom is the variant of the axiom of counting which asserts that no finite set is smaller than its own set of singletons. This paper shows that proves the consistency of the simple theory of types with infinity (). This result implies that proves that consistency of, and that proves the consistency (...)
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  • Algebraic new foundations.Paul K. Gorbow - 2019 - Journal of Symbolic Logic 84 (2):798-832.
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  • Largest initial segments pointwise fixed by automorphisms of models of set theory.Ali Enayat, Matt Kaufmann & Zachiri McKenzie - 2018 - Archive for Mathematical Logic 57 (1-2):91-139.
    Given a model \ of set theory, and a nontrivial automorphism j of \, let \\) be the submodel of \ whose universe consists of elements m of \ such that \=x\) for every x in the transitive closure of m ). Here we study the class \ of structures of the form \\), where the ambient model \ satisfies a frugal yet robust fragment of \ known as \, and \=m\) whenever m is a finite ordinal in the sense (...)
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  • Initial self-embeddings of models of set theory.Ali Enayat & Zachiri Mckenzie - 2021 - Journal of Symbolic Logic 86 (4):1584-1611.
    By a classical theorem of Harvey Friedman, every countable nonstandard model $\mathcal {M}$ of a sufficiently strong fragment of ZF has a proper rank-initial self-embedding j, i.e., j is a self-embedding of $\mathcal {M}$ such that $j[\mathcal {M}]\subsetneq \mathcal {M}$, and the ordinal rank of each member of $j[\mathcal {M}]$ is less than the ordinal rank of each element of $\mathcal {M}\setminus j[\mathcal {M}]$. Here, we investigate the larger family of proper initial-embeddings j of models $\mathcal {M}$ of fragments of (...)
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