We prove that a enumerable degree is contiguous iff it is locally distributive. This settles a twenty-year old question going back to Ladner and Sasso. We also prove that strong contiguity and contiguity coincide, settling a question of the first author, and prove that no $m$-topped degree is contiguous, settling a question of the first author and Carl Jockusch [11]. Finally, we prove some results concerning local distributivity and relativized weak truth table reducibility.

We show that Sacks' and Shoenfield's analogs of jump inversion fail for both tt- and wtt-reducibilities in a strong way. In particular we show that there is a ${\mathrm{\Delta }}_{2}^{0}$ set B > tt ∅′ such that there is no c.e. set A with A′ ≡ wtt B. We also show that there is a ${\mathrm{\Sigma }}_{2}^{0}$ set C > tt ∅′ such that there is no ${\mathrm{\Delta }}_{2}^{0}$ set D with D′ ≡ wtt C.

In this paper, we study the lattice of r.e. subspaces of a recursively presented vector space V ∞ with regard to the various complexity-theoretic speed-up properties such as speedable, effectively speedable, levelable, and effectively levelable introduced by Blum and Marques. In particular, we study the interplay between an r.e. basis A for a subspace V of V ∞ and V with regard to these properties. We show for example that if A or V is speedable , then V is levelable (...) . We also show that if A is an r.e. subset of a recursive basis for V ∞ , then A is levelable iff V is speedable while it is not the case that A is levelable iff V is speedable. We also provide several contrasts between the lattice of r.e. subspaces and the lattice of r.e. sets with respect to these speed-up properties. For example, every maximal set is levelable but we show that there exist supermaximal spaces which are nonspeedable in all possible r.e. degrees as well as supermaximal spaces which are levelable in all r.e. degrees which contain levelable sets. (shrink)

We study the relationship between a computably enumerable real and its presentations. A set A presents a computably enumerable real α if A is a computably enumerable prefix-free set of strings such that equation image. Note that equation image is precisely the measure of the set of reals that have a string in A as an initial segment. So we will simply abbreviate equation image by μ. It is known that whenever A so presents α then A ≤wttα, where ≤wtt (...) denotes weak truth table reducibility, and that the wtt-degrees of presentations form an ideal ℐ in the computably enumerable wtt-degrees. We prove that any such ideal is equation image, and conversely that if ℐ is any nonempty equation image ideal in the computably enumerable wtt-degrees then there is a computable enumerable real α such that ℐ = ℐ. We also prove a kind of Rice Theorem for these ideals, namely that if the index set of such a equation image ideal is not empty or equal to ω then it is equation image-complete. (shrink)

Downey and Remmel have completely characterized the degrees of c.e. bases for c.e. vector spaces (and c.e. fields) in terms of weak truth table degrees. In this paper we obtain a structural result concerning the interaction between the c.e. Turing degrees and the c.e. weak truth table degrees, which by Downey and Remmel's classification, establishes the existence of c.e. vector spaces (and fields) with the strong antibasis property (a question which they raised). Namely, we construct c.e. sets $B<_{\rm T}A$ such (...) that the c.e. W-degrees below that of A are disjoint from the nonzero c.e. T-degrees below that of A and comparable to that of B. (shrink)