Oscillating subalgebras of the atomless countable Boolean algebra
We show that the big Ramsey degree of the Boolean algebra with 3 atoms within the countable atomless Boolean algebra is infinite.
math.LO↗
arXiv subjects
Barbara Csima
Publications and source records attributed to Barbara Csima.
On the computability of optimal Scott sentences
Given a countable mathematical structure, its Scott sentence is a sentence of the infinitary logic $\mathcal{L}_{\omega_1 \omega}$ that characterizes it among all countable structures. We can measure the complexity of a structure by the least complexity of a Scott sentence for that structure. It is known that there can be a difference between the least complexity of a Scott sentence and the least complexity of a computable Scott sentence; for example, Alvir, Knight, and McCoy showed that there is a computable structure with a $\Pi_2$ Scott sentence but no computable $\Pi_2$ Scott sentence. It is well known that a structure with a $\Pi_2$ Scott sentence must have a computable $\Pi_4$ Scott sentence. We show that this is best possible: there is a computable structure with a $\Pi_2$ Scott sentence but no computable $\Sigma_4$ Scott sentence. We also show that there is no reasonable characterization of the computable structures with a computable $\Pi_n$ Scott sentence by showing that the index set of such structures is $\Pi^1_1$-$m$-complete.
math.LO↗
Some Questions of Uniformity in Algorithmic Randomness
The $\Omega$ numbers-the halting probabilities of universal prefix-free machines-are known to be exactly the Martin-L{\"o}f random left-c.e. reals. We show that one cannot uniformly produce, from a Martin-L{\"o}f random left-c.e. real $\alpha$, a universal prefix-free machine U whose halting probability is $\alpha$. We also answer a question of Barmpalias and Lewis-Pye by showing that given a left-c.e. real $\alpha$, one cannot uniformly produce a left-c.e. real $\beta$ such that $\alpha$ -- $\beta$ is neither left-c.e. nor right-c.e.
cs.LO↗
Positive enumerable functors
We study reductions well suited to compare structures and classes of structures with respect to properties based on enumeration reducibility. We introduce the notion of a positive enumerable functor and study the relationship with established reductions based on functors and alternative definitions.
math.LO↗
Degrees of categoricity on a cone
We investigate the complexity of isomorphisms of computable structures on cones in the Turing degrees. We show that, on a cone, every structure has a strong degree of categoricity, and that degree of categoricity is $\bf{0^{(\alpha)}}$ for some $\alpha$. To prove this, we extend Montalb\'an's $\eta$-system framework to deal with limit ordinals in a more general way. We also show that, for any fixed computable structure, there is an ordinal $\alpha$ and a cone in the Turing degrees such that the exact complexity of computing an isomorphism between the given structure and another copy $\mathcal{B}$ in the cone is a c.e. degree in $\Delta^0_\alpha(\mathcal{B})$. In each of our theorems the cone in question is clearly described in the beginning of the proof, so it is easy to see how the theorems can be viewed as general theorems with certain effectiveness conditions.
math.LO↗