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We study and compare two combinatorial lowness notions: strong jump-traceability and well-approximability of the jump, by strengthening the notion of jump-traceab-ility and super-lowness for sets of natural numbers. A computable non-decreasing unbounded function h is called an order function. Informally, a set A is strongly jump-traceable if for each order… (More)

Dickson's Lemma is a simple yet powerful tool widely used in decidabil-ity proofs, especially when dealing with counters or related data structures in algorithmics, verification and model-checking, constraint solving, logic, etc. While Dickson's Lemma is well-known, most computer scientists are not aware of the complexity upper bounds that are entailed by… (More)

Consider a Martin-Löf random ∆ 0 2 set Z. We give lower bounds for the number of changes of Zs n for computable approximations of Z. We show that each nonempty Π 0 1 class has a low member Z with a computable approximation that changes only o(2 n) times. We prove that each superlow ML-random set already satisfies a stronger randomness notion called balanced… (More)

Well quasi-orders (wqo's) are an important mathematical tool for proving termination of many algorithms. Under some assumptions upper bounds for the computational complexity of such algorithms can be extracted by analyzing the length of controlled bad sequences. We develop a new, self-contained study of the length of bad sequences over the product ordering… (More)

We define the notion of indifferent set with respect to a given class of {0, 1}-sequences. Roughly, for a set A in the class, a set of natural numbers I is indifferent for A with respect to the class if it does not matter how we change A at the positions in I: the new sequence continues to be in the given class. We are especially interested in studying… (More)

The first example of an absolutely normal number was given by Sierpinski in 1916, twenty years before the concept of computability was formalized. In this note we give a recursive reformulation of Sierpinski's construction which produces a computable absolutely normal number.

The present work investigates several questions from a recent survey of Miller and Nies related to Chaitin's Ω numbers and their dependence on the underlying universal machine. It is shown that there are universal machines for which Ω U is just x 2 1−H(x). For such a universal machine there exists a co-r.e. set X such that Ω U [X] = p:U (p)↓∈X 2 −|p| is… (More)

In an unpublished manuscript, Alan Turing gave a computable construction to show that absolutely normal real numbers between 0 and 1 have Lebesgue measure 1; furthermore, he gave an algorithm for computing instances in this set. We complete his manuscript by giving full proofs and correcting minor errors. While doing this, we recreate Turing's ideas as… (More)

Taking as inspiration the hybrid logic HL(↓), we introduce a new family of logics that we call memory logics. In this article we present in detail two interesting members of this family defining their formal syntax and semantics. We then introduce a proper notion of bisimulation and investigate their expressive power (in comparison with modal and hybrid… (More)

We consider the question of randomness of the probability Ω U [X] that an optimal Turing machine U halts and outputs a string in a fixed set X.