Kolmogorov Complexity and Instance Complexity of Recursively Enumerable Sets

We study in which way Kolmogorov complexity and instance complexity affect properties of r.e. sets. We show that the well-known 2 log n upper bound on the Kolmogorov complexity of initial segments of r.e. sets is optimal and characterize the T-degrees of r.e. sets which attain this bound. The main part of the paper is concerned with instance complexity of r.e. sets. We construct a nonrecursive r.e. set with instance complexity logarithmic in the Kolmogorov complexity. This refutes a conjecture of Ko, Orponen, Schöning, and Watanabe. In the other extreme, we show that all wtt-complete set and all Q-complete sets have infinitely many hard instances.