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Non-branching degrees in the Medvedev lattice of Π10 classes

  • Christopher P. Alfeld (a1)


A class is the set of paths through a computable tree. Given classes P and Q, P is Medvedev reducible to Q, PMQ, if there is a computably continuous functional mapping Q into P. We look at the lattice formed by subclasses of 2ω under this reduction. It is known that the degree of a splitting class of c.e. sets is non-branching. We further characterize non-branching degrees, providing two additional properties which guarantee non-branching: inseparable and hyperinseparable. Our main result is to show that non-branching iff inseparable if hyperinseparable if homogeneous and that all unstated implications do not hold. We also show that inseparable and not hyperinseparable degrees are downward dense.



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