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There in no sw-complete c.e. real. (English) Zbl 1070.03028
A real number \(\alpha\) is called random if for every \(n\), the sequence \(\alpha_{| n}\) of its first \(n\) binary digits cannot be compressed, i.e., if \(K(\alpha_{| n})\geq n-O(1)\), where \(K(z)\) is a prefix-free Kolmogorov complexity of the sequence \(z\) (i.e., crudely speaking, the shortest length of a program that computes a binary sequence starting with \(z\)).
For computably enumerable (c.e.) real numbers, we can define when one of them is “more random” than another: \(\alpha\leq _K \beta\) if and only if \(K(\alpha_{| n})\leq K(\beta_{| n})+O(1)\). It is known that Chaitin’s \(\Omega\) (the halting probability of a universal prefix-free Turing machine) is random and c.e. and it is “more random” than any other c.e. real \(\beta\) in the sense that \(\beta\leq _K \Omega\).
It is difficult to compute Kolmogorov complexity, so a new definition of sw-reducibility (strongly weak truth table reducibility) \(\leq_{\text{sw}}\) was proposed. It is known that sw-reducibility implies \(\beta \leq_K\alpha\). Because of this, it was conjectured that there exists an sw-complete c.e. real number, i.e., a c.e. real number \(\alpha\) such that \(\beta \leq_{\text{sw}}\alpha\) for all c.e. real numbers \(\beta\). The authors prove that there is no such number; moreover, they prove that there exist two non-random c.e. reals \(\beta_0\) and \(\beta_1\) for which no c.e. real \(\alpha\) exists for which \(\beta_0\leq_{\text{sw}}\alpha\) and \(\beta_1\leq _{\text{sw}}\alpha\).

MSC:
03D80 Applications of computability and recursion theory
68Q30 Algorithmic information theory (Kolmogorov complexity, etc.)
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