Abstract:
Every infinite binary sequence is Turing reducible to a random one. This is
a corollary of a result of Peter Gacs stating that for every co-r.e. closed set with
positive measure of infinite sequences there exists a computable mapping which
maps a subset of the set onto the whole space of infinite sequences. Cristian
Calude asked whether in this result one can replace the positive measure condition by a weaker condition not involving the measure. We show that this is
indeed possible: it is sufficient to demand that the co-r.e. closed set contains a
computably growing Cantor set. Furthermore, in the case of a set with positive
measure we construct a surjective computable map which is more effective than
the map constructed by Gacs.
