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SAS

Seminar

SJT-hardness and pseudo-jump inversion

Turetsky, D (Victoria University of Wellington)
Thursday 05 July 2012, 09:00-10:00

Seminar Room 1, Newton Institute

Abstract

Tracing was introduced to computability theory by Terwijn and Zambella, who used it to characterize the degrees which are low for Schnorr randomness. Zambella observed that tracing also has a relationship to K-triviality, which was strengthened by Nies and then later others. A trace for a partial function f is a sequence of finite sets (T_z) with f(z) in T_z for all z in the domain of f. A trace (T_z) is c.e. if the T_z are uniformly c.e. sets. An order function is a total, nondecreasing, unbounded positive function. If h is an order, a trace (T_z) is an h-trace if h(z) bounds the size of T_z. A set is called jump-traceable (JT) if every partial function it computes has a c.e. h-trace for some computable order h. A set is called strongly jump-traceable (SJT) if every partial function it computes has a c.e. h-trace for every computable order h. Figuiera et al. constructed a non-computable c.e. set which is SJT. This gives a natural pseudo-jump operator. Pseudo-jump inverting this SJT-operator gives a set A such that the halting problem looks SJT relative to A. That is, for every partial function computable from the halting problem, and every computable order h, there is an h-trace which is uniformly c.e. relative to A. Such a set is called SJT-hard. Downey and Greenberg showed that there is a non-computable c.e. set E which is computable from every c.e. SJT-hard set. Thus the SJT-operator cannot be pseudo-jump inverted outside of the cone above E. We strengthen this result, showing that E can be made super high.

Presentation

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