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  1. How Incomputable Is the Separable Hahn-Banach Theorem?Guido Gherardi & Alberto Marcone - 2009 - Notre Dame Journal of Formal Logic 50 (4):393-425.
    We determine the computational complexity of the Hahn-Banach Extension Theorem. To do so, we investigate some basic connections between reverse mathematics and computable analysis. In particular, we use Weak König's Lemma within the framework of computable analysis to classify incomputable functions of low complexity. By defining the multivalued function Sep and a natural notion of reducibility for multivalued functions, we obtain a computational counterpart of the subsystem of second-order arithmetic WKL0. We study analogies and differences between WKL0 and the class (...)
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  • Randomness, relativization and Turing degrees.André Nies, Frank Stephan & Sebastiaan A. Terwijn - 2005 - Journal of Symbolic Logic 70 (2):515-535.
    We compare various notions of algorithmic randomness. First we consider relativized randomness. A set is n-random if it is Martin-Löf random relative to ∅. We show that a set is 2-random if and only if there is a constant c such that infinitely many initial segments x of the set are c-incompressible: C ≥ |x|-c. The ‘only if' direction was obtained independently by Joseph Miller. This characterization can be extended to the case of time-bounded C-complexity. Next we prove some results (...)
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  • Baire Categoricity and $\Sigma^{0}_{1}$ -Induction.Stephen G. Simpson - 2014 - Notre Dame Journal of Formal Logic 55 (1):75-78.
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  • On the (semi)lattices induced by continuous reducibilities.Arno Pauly - 2010 - Mathematical Logic Quarterly 56 (5):488-502.
    Continuous reducibilities are a proven tool in Computable Analysis, and have applications in other fields such as Constructive Mathematics or Reverse Mathematics. We study the order-theoretic properties of several variants of the two most important definitions, and especially introduce suprema for them. The suprema are shown to commutate with several characteristic numbers.
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  • Algorithmic randomness, reverse mathematics, and the dominated convergence theorem.Jeremy Avigad, Edward T. Dean & Jason Rute - 2012 - Annals of Pure and Applied Logic 163 (12):1854-1864.
    We analyze the pointwise convergence of a sequence of computable elements of L1 in terms of algorithmic randomness. We consider two ways of expressing the dominated convergence theorem and show that, over the base theory RCA0, each is equivalent to the assertion that every Gδ subset of Cantor space with positive measure has an element. This last statement is, in turn, equivalent to weak weak Königʼs lemma relativized to the Turing jump of any set. It is also equivalent to the (...)
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  • The Bolzano–Weierstrass Theorem is the jump of Weak Kőnig’s Lemma.Vasco Brattka, Guido Gherardi & Alberto Marcone - 2012 - Annals of Pure and Applied Logic 163 (6):623-655.
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  • Weihrauch degrees, omniscience principles and weak computability.Vasco Brattka & Guido Gherardi - 2011 - Journal of Symbolic Logic 76 (1):143 - 176.
    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 semi-lattice. It turns out that parallelization is a closure operator for this semi-lattice and that the parallelized Weihrauch degrees even form a lattice into which the Medvedev lattice and the Turing degrees can be embedded. The (...)
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  • The baire category theorem in weak subsystems of second-order arithmetic.Douglas K. Brown & Stephen G. Simpson - 1993 - Journal of Symbolic Logic 58 (2):557-578.
    Working within weak subsystems of second-order arithmetic Z2 we consider two versions of the Baire Category theorem which are not equivalent over the base system RCA0. We show that one version (B.C.T.I) is provable in RCA0 while the second version (B.C.T.II) requires a stronger system. We introduce two new subsystems of Z2, which we call RCA+ 0 and WKL+ 0, and show that RCA+ 0 suffices to prove B.C.T.II. Some model theory of WKL+ 0 and its importance in view of (...)
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  • Closed choice and a uniform low basis theorem.Vasco Brattka, Matthew de Brecht & Arno Pauly - 2012 - Annals of Pure and Applied Logic 163 (8):986-1008.
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  • Denis R. Hirschfeldt. Slicing the truth: On the computable and reverse mathematics of combinatorial principles. Lecture Note Series, Institute for Mathematical Sciences, National University of Singapore, vol. 28. World Scientific Publishing Co. Pte. Ltd., Singapore, 2015, xiv+214 pp. [REVIEW]Jeffry L. Hirst - 2015 - Bulletin of Symbolic Logic 21 (3):338-339.
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  • Effective Borel measurability and reducibility of functions.Vasco Brattka - 2005 - Mathematical Logic Quarterly 51 (1):19-44.
    The investigation of computational properties of discontinuous functions is an important concern in computable analysis. One method to deal with this subject is to consider effective variants of Borel measurable functions. We introduce such a notion of Borel computability for single-valued as well as for multi-valued functions by a direct effectivization of the classical definition. On Baire space the finite levels of the resulting hierarchy of functions can be characterized using a notion of reducibility for functions and corresponding complete functions. (...)
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  • Effective choice and boundedness principles in computable analysis.Vasco Brattka & Guido Gherardi - 2011 - Bulletin of Symbolic Logic 17 (1):73-117.
    In this paper we study a new approach to classify mathematical theorems according to their computational content. Basically, we are asking the question which theorems can be continuously or computably transferred into each other? For this purpose theorems are considered via their realizers which are operations with certain input and output data. The technical tool to express continuous or computable relations between such operations is Weihrauch reducibility and the partially ordered degree structure induced by it. We have identified certain choice (...)
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