Double jumps of minimal degrees over cardinals

Extract

Jockusch and Posner [4] showed that every minimal ω-degree is GL₂. This is achieved by exhibiting a function f recursive in 0′ which dominates every function of minimal ω-degree. The function f has the peculiar property that for every s, f(s) is defined after a search (using 0′) over the power set of L_s (Gödel's constructible hierarchy at the level s). It can be seen that a function defined in a similar manner over an infinite successor cardinal k will not be a total function, since for example if k = ρ⁺, then f(ρ) will not be defined until after all the subsets of ρ have been examined, and this will take at least k steps. The following questions then naturally arise: (i) For successor cardinals k, is there a function dominating every set of minimal k-degree? (ii) For arbitrary cardinals k, is every minimal k-degree GL₂ (i.e. b″ = (b ∨ 0′) for b of minimal k-degree)? In this paper we answer (i) in the negative and provide a positive answer to (ii), assuming V = L. We show in fact that if k is a successor cardinal and h ≤_k 0′, then there is a function of minimal k-degree below 0′ not dominated by h (Theorem 1). This implies that any refinement of the function f described above will not be able to remove the difficulties encountered. On the other hand, we introduce the notion of ‘strong domination’ to provide a positive answer to (ii) (Theorem 2 and Corollary 1). We end this paper by indicating that for limit cardinals k, there is a function below 0′ dominating every function of minimal k-degree.