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Avoiding effective packing dimension 1 below array noncomputable c.e. degrees. (English) Zbl 1415.03047
The present article is concerned with the notion of effective packing dimension when applied to Turing lower cones. L. Bienvenu et al. [Theory Comput. Syst. 45, No. 4, 740–755 (2009; Zbl 1183.68281)] and L. Fortnow et al. [Lect. Notes Comput. Sci. 4051, 335–345 (2006; Zbl 1223.68060)] have independently shown that when taking the supremum over the effective packing dimensions of all sets in the Turing lower cone of a set \(X\), then that number must either be 0 or 1. This behaviour is notable as it is in stark contrast to that observed by J. S. Miller [Adv. Math. 226, No. 1, 373–384 (2011; Zbl 1214.03030)] for the closely related notion of effective Hausdorff dimension.
An obvious example for when the supremum takes the value 0 is if \(X\) is computable: then all sets in \(X\)’s lower cone are also computable, and all of them have effective packing dimension 0. For the value 1, there are two possible cases: The supremum might be an attained maximum over the lower cone; or it might be an unattained supremum. Clearly, lower cones of the first type exist: if \(X\) is Martin-Löf random, \(X\) has effective packing dimension 1. C. J. Conidis [J. Symb. Log. 77, No. 2, 447–474 (2012; Zbl 1251.03047)] showed that lower cones of the second type exist as well.
A natural question was then to ask which lower cones exactly are of the second type. The authors precisely answer this question for lower cones that are below the halting problem. A result of M. Kummer [SIAM J. Comput. 25, No. 6, 1123–1143 (1996; Zbl 0859.03015)] suggest that array noncomputability may have a role to play in this particular case. And indeed, within the c.e. sets, the present article characterizes the array noncomputable sets as exactly those below which a lower cone of the second type can be found.
The result is proved using pruned clumpy trees, building on the notion of clumpy trees introduced by R. Downey and N. Greenberg [Inf. Process. Lett. 108, No. 5, 298–303 (2008; Zbl 1191.68304)].

03D32 Algorithmic randomness and dimension
03D28 Other Turing degree structures
68Q30 Algorithmic information theory (Kolmogorov complexity, etc.)
Full Text: DOI
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