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  1. Embeddings between well-orderings: Computability-theoretic reductions.Jun Le Goh - 2020 - Annals of Pure and Applied Logic 171 (6):102789.
    We study the computational content of various theorems with reverse mathematical strength around Arithmetical Transfinite Recursion (ATR_0) from the point of view of computability-theoretic reducibilities, in particular Weihrauch reducibility. Our main result states that it is equally hard to construct an embedding between two given well-orderings, as it is to construct a Turing jump hierarchy on a given well-ordering. This answers a question of Marcone. We obtain a similar result for Fraïssé's conjecture restricted to well-orderings.
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  • Provable Better-Quasi-Orders.Anton Freund, Alberto Marcone, Fedor Pakhomov & Giovanni Soldà - 2025 - Notre Dame Journal of Formal Logic -1:1-14.
    It has recently been shown that fairly strong axiom systems such as ACA0 cannot prove that the antichain with three elements is a better-quasi-order (bqo). In the present paper, we give a complete characterization of the finite partial orders that are provably bqo in such axiom systems. The result will also be extended to infinite orders. As an application, we derive that a version of the minimal bad array lemma is weak over ACA0. In sharp contrast, a recent result shows (...)
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  • Weak Well Orders and Fraïssé’s Conjecture.Anton Freund & Davide Manca - forthcoming - Journal of Symbolic Logic:1-16.
    The notion of countable well order admits an alternative definition in terms of embeddings between initial segments. We use the framework of reverse mathematics to investigate the logical strength of this definition and its connection with Fraïssé’s conjecture, which has been proved by Laver. We also fill a small gap in Shore’s proof that Fraïssé’s conjecture implies arithmetic transfinite recursion over $\mathbf {RCA}_0$, by giving a new proof of $\Sigma ^0_2$ -induction.
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