On a hierarchy of groups of computable automorphisms.
On dot-depth two
On interpretability in set theories. II.
On nonmonotone inductive definability
On the hierarchies of Δ20-real numbers
A real number x is called Δ20 if its binary expansion corresponds to a Δ20-set of natural numbers. Such reals are just the limits of computable sequences of rational numbers and hence also called computably approximable. Depending on how fast the sequences converge, Δ20-reals have different levels of effectiveness. This leads to various hierarchies of Δ20 reals. In this survey paper we summarize several recent developments related to such kind of hierarchies shown by the author and his collaborators. ...
On the topological complexity of infinitary rational relations
We prove in this paper that there exists some infinitary rational relations which are analytic but non Borel sets, giving an answer to a question of Simonnet [20].
On the Topological Complexity of Infinitary Rational Relations
We prove in this paper that there exists some infinitary rational relations which are analytic but non Borel sets, giving an answer to a question of Simonnet [20].
On two complete sets in the analytical and the arithmetical hierarchies.
Optimal bounds for ordinal comparison maps.