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  1.  16
    Cupping and Noncupping in the Enumeration Degrees of ∑20 Sets.S. Barry Cooper, Andrea Sorbi & Xiaoding Yi - 1996 - Annals of Pure and Applied Logic 82 (3):317-342.
    We prove the following three theorems on the enumeration degrees of ∑20 sets. Theorem A: There exists a nonzero noncuppable ∑20 enumeration degree. Theorem B: Every nonzero Δ20enumeration degree is cuppable to 0′e by an incomplete total enumeration degree. Theorem C: There exists a nonzero low Δ20 enumeration degree with the anticupping property.
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  2.  24
    On the Distribution of Lachlan Nonsplitting Bases.S. Barry Cooper, Angsheng Li & Xiaoding Yi - 2002 - Archive for Mathematical Logic 41 (5):455-482.
    We say that a computably enumerable (c.e.) degree b is a Lachlan nonsplitting base (LNB), if there is a computably enumerable degree a such that a > b, and for any c.e. degrees w,v ≤ a, if a ≤ w or; v or; b then either a ≤ w or; b or a ≤ v or; b. In this paper we investigate the relationship between bounding and nonbounding of Lachlan nonsplitting bases and the high /low hierarchy. We prove that there (...)
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  3.  17
    Cupping and Noncupping in the Enumeration Degrees of∑< Sub> 2< Sup> 0 Sets.S. Barry Cooper, Andrea Sorbi & Xiaoding Yi - 1996 - Annals of Pure and Applied Logic 82 (3):317-342.
  4.  13
    The Discontinuity of Splitting in the Recursively Enumerable Degrees.S. Barry Cooper & Xiaoding Yi - 1995 - Archive for Mathematical Logic 34 (4):247-256.
    In this paper we examine a class of pairs of recursively enumerable degrees, which is related to the Slaman-Soare Phenomenon.
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  5.  11
    Jump Theorems for REA Operators.Alistair H. Lachlan & Xiaoding Yi - 1993 - Mathematical Logic Quarterly 39 (1):1-6.
    In [2], Jockusch and Shore have introduced a new hierarchy of sets and operators called the REA hierarchy. In this note we prove analogues of the Friedberg Jump Theorem and the Sacks Jump Theorem for many REA operators. MSC: 03D25, 03D55.
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  6.  33
    A Non-Splitting Theorem for D.R.E. Sets.Xiaoding Yi - 1996 - Annals of Pure and Applied Logic 82 (1):17-96.
    A set of natural numbers is called d.r.e. if it may be obtained from some recursively enumerable set by deleting the numbers belonging to another recursively enumerable set. Sacks showed that for each non-recursive recursively enumerable set A there are disjoint recursively enumerable sets B, C which cover A such that A is recursive in neither A ∩ B nor A ∩ C. In this paper, we construct a counterexample which shows that Sacks's theorem is not in general true when (...)
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  7.  7
    Splittings of 0' Into the Recursively Enumerable Degrees.Xiaoding Yi - 1996 - Mathematical Logic Quarterly 42 (1):249-269.
    Lachlan [9] proved that there exists a non-recursive recursively enumerable degree such that every non-recursive r. e. degree below it bounds a minimal pair. In this paper we first prove the dual of this fact. Second, we answer a question of Jockusch by showing that there exists a pair of incomplete r. e. degrees a0, a1 such that for every non-recursive r. e. degree w, there is a pair of incomparable r. e. degrees b0, b2 such that w = b0 (...)
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