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Embeddings between well-orderings: computability-theoretic reductions. (English) Zbl 07189161
Summary: We study the computational content of various theorems with reverse mathematical strength around Arithmetical Transfinite Recursion $$(\mathsf{ATR}_0)$$ 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.
##### MSC:
 03B30 Foundations of classical theories (including reverse mathematics) 03D30 Other degrees and reducibilities in computability and recursion theory 03D80 Applications of computability and recursion theory 03F35 Second- and higher-order arithmetic and fragments 03D55 Hierarchies of computability and definability
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