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**Weihrauch degrees, omniscience principles and weak computability.**
*(English)*
Zbl 1222.03071

This paper is a sequel to [G. Gherardi and A. Marcone, Notre Dame J. Formal Logic 50, No. 4, 393–425 (2009; Zbl 1223.03052)]. The abstract that the authors give is very adequate: “In this paper we study a reducibility that has been introduced by Klaus Weihrauch or, more precisely, a natural extension for multi-valued functions on represented spaces. We call the corresponding equivalence classes Weihrauch degrees and we show that the corresponding partial order induces a lower semilattice. It turns out that parallelization is a closure operator for this semilattice and that the parallelized Weihrauch degrees even form a lattice into which the Medvedev lattice and the Turing degrees can be embedded. The importance of Weihrauch degrees is based on the fact that multi-valued functions on represented spaces can be considered as realizers of mathematical theorems in a very natural way and studying the Weihrauch reductions between theorems in this sense means to ask which theorems can be transformed continuously or computably into each other. As crucial corner points of this classification scheme the limited principle of omniscience, LPO, the lesser limited principle of omniscience, LLPO, and their parallelizations are studied. It is proved that parallelized LLPO is equivalent to Weak König’s Lemma and hence to the Hahn-Banach Theorem in this new and very strong sense. We call a multi-valued function weakly computable if it is reducible to the Weihrauch degree of parallelized LLPO and we present a new proof, based on a computational version of Kleene’s ternary logic, that the class of weakly computable operations is closed under composition. Moreover, weakly computable operations on computable metric spaces are characterized as operations that admit upper semi-computable compact-valued selectors and it is proved that any single-valued weakly computable operation is already computable in the ordinary sense.”

Reviewer: Iris Loeb (Christchurch)

### MSC:

03F60 | Constructive and recursive analysis |

03B30 | Foundations of classical theories (including reverse mathematics) |

03D30 | Other degrees and reducibilities in computability and recursion theory |

03D45 | Theory of numerations, effectively presented structures |

03E15 | Descriptive set theory |

### Keywords:

computable analysis; constructive analysis; reverse mathematics; effective descriptive set theory### Citations:

Zbl 1223.03052
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\textit{V. Brattka} and \textit{G. Gherardi}, J. Symb. Log. 76, No. 1, 143--176 (2011; Zbl 1222.03071)

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