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Abstract: In this paper we introduce the notion of ε-universal prefix-free Turing machine (ε is a computable real in ) and study its halting probability. The main result is the extension of the representability theorem for left-computable random reals to the case of ε-random reals: a real is left-computable ε-random iff it is the halting probability of an ε-universal prefix-free Turing machine. We also show that left-computable ε-random reals are provable ε-random in the Peano Arithmetic. The theory developed here parallels to a large extent the classical theory, but not completely. For example, random reals are Borel normal (in any base), but for , some ε-random reals do not contain even arbitrarily long runs of 0s. [Copyright &y& Elsevier]

Abstract: Universality, provability and simplicity are key notions in computability theory. There are various criteria of simplicity for universal Turing machines. Probably the most popular one is to count the number of states/symbols. This criterion is more complex than it may appear at a first glance. In this note we propose three new criteria of simplicity for universal prefix-free Turing machines. These criteria refer to the possibility of proving various natural properties of such a machine (its universality, for example) in a formal theory, Peano arithmetic or Zermelo–Fraenkel set theory. In all cases some, but not all, machines are simple. [ABSTRACT FROM AUTHOR]