We characterize Δ20-categoricity in Boolean algebras and linear orderings under some extra effectiveness conditions. We begin with a study of the relativized notion in these structures.

The main result is that for sets ${S \subseteq \omega + 1}$ , the following are equivalent: The shuffle sum σ(S) is computable.The set S is a limit infimum set, i.e., there is a total computable function g(x, t) such that ${f(x) = \lim inf_t g(x, t)}$ enumerates S.The set S is a limitwise monotonic set relative to 0′, i.e., there is a total 0′-computable function ${\tilde{g}(x, t)}$ satisfying ${\tilde{g}(x, t) \leq \tilde{g}(x, t+1)}$ such that ${{\tilde{f}(x) = \lim_t \tilde{g}(x, t)}}$ (...) enumerates S.Other results discuss the relationship between these sets and the ${\Sigma^0_3}$ sets. (shrink)

We show that a set has an η-representation in a linear order if and only if it is the range of a 0'-computable limitwise monotonic function. We also construct a Δ₃ Turing degree for which no set in that degree has a strong η-representation, answering a question posed by Downey.

In this paper we prove that any Δ30 degree has an increasing η -representation. Therefore, there is an increasing η -representable set without a strong η -representation.

Defining a class of sets to be uniform Δ02 if it is derived from a binary {0,1}{0,1}-valued function f≤TKf≤TK, we show that, for any C⊆DeC⊆De induced by such a class, there exists a high Δ02 degree c which is incomparable with every degree b ϵ Ce \ {0e, 0'e}. We show how this result can be applied to quite general subclasses of the Ershov Hierarchy and we also prove, as a direct corollary, that every nonzero low degree caps with both (...) a high and a nonzero low Δ02 degree. (shrink)