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Low upper bounds in the LR degrees. (English) Zbl 1247.03073
Summary: We say that $$A\leq _{\mathrm {LR}} B$$ if every $$B$$-random real is $$A$$-random – in other words, if $$B$$ has at least as much derandomization power as $$A$$. The LR reducibility is a natural weak reducibility in the context of randomness, and generalizes lowness for randomness. We study the existence and properties of upper bounds in the context of the LR degrees. In particular, we show that given two (or even finitely many) low sets, there is a low c.e. set which lies LR above both. This is very different from the situation in the Turing degrees, where Sacks’ splitting theorem shows that two low sets can join to $$\mathbf {0^{\prime}}$$.

##### MSC:
 03D25 Recursively (computably) enumerable sets and degrees 03D30 Other degrees and reducibilities in computability and recursion theory 03D32 Algorithmic randomness and dimension 68Q30 Algorithmic information theory (Kolmogorov complexity, etc.)
##### Keywords:
computability theory; randomness; K-trivials; lowness
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