Published online by Cambridge University Press: 17 April 2014
By the complexity of a finite sequence of 0’s and 1’s wemean the Kolmogorov complexity, that is the length of the shortest input to auniversal recursive function which returns the given sequence as output. Byinitial segment complexity of an infinite sequence of 0’s and1’s we mean the asymptotic behavior of the complexity of its finiteinitial segments. In this paper, we construct infinite sequences of0’s and 1’s with given recursive lower bounds on initialsegment complexity which do not compute any infinite sequences of 0’sand 1’s with a significantly larger recursive lower bound on initialsegment complexity. This improves several known results about randomnessextraction and separates many natural degrees in the lattice of Muchnikdegrees.