Guessing, Mind-changing, and the Second Ambiguous Class
  In his dissertation, Wadge defined a notion of guessability on subsets of the Baire space and gave two characterizations of guessable sets. A set is guessable iff it is in the second ambiguous class (boldface Delta^0_2), iff it is eventually annihilated by a certain remainder. We simplify this remainder and give a new proof of the latter equivalence. We then introduce a notion of guessing with an ordinal limit on how often one can change one's mind. We show that for every ordinal alpha, a guessable set is annihilated by alpha applications of the simplified remainder if and only if it is guessable with fewer than alpha mind changes. We use guessability with fewer than alpha mind changes to give a semi-characterization of the Hausdorff difference hierarchy, and indicate how Wadge's notion of guessability can be generalized to higher-order guessability, providing characterizations for boldface Delta^0_alpha for all successor ordinals alpha>1. 
Samuel Alexander
math.LO