Towards characterizing the >ω2-fickle recursively enumerable Turing degrees

Abstract

Given a finite lattice L that can be embedded in the recursively enumerable (r.e.) Turing degrees $〈R_T,{\le }_T〉$, it is not known how one can characterize the degrees $d\in R_T$ below which L can be embedded. Two important characterizations are of the $L_7$ and $M_3$ lattices, where the lattices are embedded below d if and only if d contains sets of “fickleness” >ω and $\ge {\omega }^{\omega }$ respectively. We work towards finding a lattice that characterizes the levels above ${\omega }^2$, the first non-trivial level after ω. We considered lattices that are as “short” in height and “narrow” in width as $L_7$ and $M_3$, but the lattices characterize also the >ω or $\ge {\omega }^{\omega }$ levels, if the lattices are not already embeddable below all non-zero r.e. degrees. We also considered upper semilattices (USLs) by removing the bottom meet(s) of some previously considered lattices, but the removals did not change the levels characterized. We discovered three lattices besides $M_3$ that also characterize the $\ge {\omega }^{\omega }$-levels. Our search for $>{\omega }^2$-candidates can therefore be reduced to the lattice-theoretic problem of finding lattices that do not contain any of the four $\ge {\omega }^{\omega }$-lattices as sublattices.