Let $T$ be a recursively enumerable theory extending Elementary Arithmetic $\rm{EA}$. L. D. Beklemishev proved that the $\Sigma_2$ local reflection principle for $T$, $\mathsf{Rfn}_{\Sigma_2}(T)$, is conservative over the $\Sigma_1$ local reflection principle, $\mathsf{Rfn}_{\Sigma_1}(T)$, with respect to boolean combinations of $\Sigma_1$-sentences; and asked whether this result is best possible. In this work we answer Beklemishev's question by showing that $\Pi_2$-sentences are not conserved for $T = \rm{EA}{}+{}\textit{"f is total"}$, where $f$ is any nondecreasing computable function with elementary graph. We also discuss how this result generalizes to $n > 0$ and obtain as an application that for $n > 0$, $I\Pi_{n+1}^-$ is conservative over $I\Sigma_n$ with respect to $\Pi_{n+2}$-sentences.

For a fixed countably infinite structure $\Gamma$ with finite relational signature $\tau$, we study the following computational problem: input are quantifier-free $\tau$-formulas $\phi_0,\phi_1,\dots,\phi_n$ that define relations $R_0,R_1,\dots,R_n$ over $\Gamma$. The question is whether the relation $R_0$ is primitive positive definable from $R_1,\dots,R_n$, i.e., definable by a first-order formula that uses only relation symbols for $R_1, \dots, R_n$, equality, conjunctions, and existential quantification (disjunction, negation, and universal quantification are forbidden).

We show decidability of this problem for all structures $\Gamma$ that have a first-order definition in an ordered homogeneous structure $\Delta$ with a finite relational signature whose age is a Ramsey class and determined by finitely many forbidden substructures. Examples of structures $\Gamma$ with this property are the order of the rationals, the random graph, the homogeneous universal poset, the random tournament, all homogeneous universal $C$-relations, and many more. We also obtain decidability of the problem when we replace primitive positive definability by existential positive, or existential definability. Our proof makes use of universal algebraic and model theoretic concepts, Ramsey theory, and a recent characterization of Ramsey classes in topological dynamics.

We say that an uncountable metric space is computably categorical if every two computable structures on this space are equivalent up to a computable isometry. We show that Cantor space, the Urysohn space, and every separable Hilbert space are computably categorical, but the space $ \mathcal{C}[0,1]$ of continuous functions on the unit interval with the supremum metric is not. We also characterize computably categorical subspaces of $\mathbb{R}^n$, and give a sufficient condition for a space to be computably categorical. Our interest is motivated by classical and recent results in computable (countable) model theory and computable analysis.

We present definitions of homology groups $H_n(p)$, $n\ge 0$, associated to a complete type $p$. We show that if the generalized amalgamation properties hold, then the homology groups are trivial. We compute the group $H_2(p)$ for strong types in stable theories and show that any profinite abelian group can occur as the group $H_2(p)$.

We prove that the determinacy of Gale-Stewart games whose winning sets are accepted by real-time 1-counter Büchi automata is equivalent to the determinacy of (effective) analytic Gale-Stewart games which is known to be a large cardinal assumption. We show also that the determinacy of Wadge games between two players in charge of $\omega$-languages accepted by 1-counter Büchi automata is equivalent to the (effective) analytic Wadge determinacy. Using some results of set theory we prove that one can effectively construct a 1-counter Büchi automaton $\mathcal{A}$ and a Büchi automaton $\mathcal{B}$ such that: (1) There exists a model of ZFC in which Player 2 has a winning strategy in the Wadge game $W(L(\mathcal{A}), L(\mathcal{B}))$; (2) There exists a model of ZFC in which the Wadge game $W(L(\mathcal{A}), L(\mathcal{B}))$ is not determined. Moreover these are the only two possibilities, i.e. there are no models of ZFC in which Player 1 has a winning strategy in the Wadge game $W(L(\mathcal{A}), L(\mathcal{B}))$.

In this paper, we investigate the existence of a Friedberg numbering in fragments of Peano Arithmetic and initial segments of Gödel's constructible hierarchy $L_\alpha$, where $\alpha$ is $\Sigma_1$ admissible. We prove that

(1) Over $P^-+B\Sigma_2$, the existence of a Friedberg numbering is equivalent to $I\Sigma_2$, and

(2) For $L_\alpha$, there is a Friedberg numbering if and only if the tame $\Sigma_2$ projectum of $\alpha$ equals the $\Sigma_2$ cofinality of $\alpha$.

We prove the consistency of $\mathfrak{b} > \aleph_1$ together with the existence of a $\Pi^1_1$-definable mad family, answering a question posed by Friedman and Zdomskyy in [7, Question 16]. For the proof we construct a mad family in $L$ which is an $\aleph_1$-union of perfect a.d. sets, such that this union remains mad in the iterated Hechler extension. The construction also leads us to isolate a new cardinal invariant, the Borel almost-disjointness number $\mathfrak{a}_B$, defined as the least number of Borel a.d. sets whose union is a mad family. Our proof yields the consistency of $\mathfrak{a}_B < \mathfrak{b}$ (and hence, $\mathfrak{a}_B < \mathfrak{a}$).

We answer in the affirmative the following question of Jörg Brendle: If there is a $\Sigma^1_2$ mad family, is there then a $\Pi^1_1$ mad family?

In 1986, Osherson, Stob and Weinstein asked whether two variants of anomalous vacillatory learning, TxtFex$^*_*$ and TxtFext$^*_*$, could be distinguished [3]. In both, a machine is permitted to vacillate between a finite number of hypotheses and to make a finite number of errors. TxtFext$^*_*$-learning requires that hypotheses output infinitely often must describe the same finite variant of the correct set, while TxtFex$^*_*$-learning permits the learner to vacillate between finitely many different finite variants of the correct set. In this paper we show that TxtFex$^*_*$ $\neq$ TxtFext$^*_*$, thereby answering the question posed by Osherson, et al. We prove this in a strong way by exhibiting a family in TxtFex$^*_2 \setminus \mbox{TxtFext}^*_*$.

For a countable structure $\mathcal{B}$, the spectrum is the set of Turing degrees of isomorphic copies of $\mathcal{B}$. For a complete elementary first order theory $T$, the spectrum is the set of Turing degrees of models of $T$. We answer a question from [1] by showing that there is an atomic theory $T$ whose spectrum does not match the spectrum of any structure.

We introduce the notions of copyable and diagonalizable classes of structures. We then show how these notions are connected to two other notions that had already been studied for some particular classes of structures, namely the listability property and the low property.

The main result of this paper is the characterizations of the classes of structures with the low property, that is, the classes whose low members all have computable copies. We characterize these classes as the ones whose structural jumps are listable.

An infinite binary sequence $A$ is absolutely undecidable if it is impossible to compute $A$ on a set of positions of positive upper density. Absolute undecidability is a weakening of bi-immunity. Downey, Jockusch and Schupp [2] asked whether, unlike the case for bi-immunity, there is an absolutely undecidable set in every non-zero Turing degree. We provide a positive answer to this question by applying techniques from coding theory. We show how to use Walsh—Hadamard codes to build a truth-table functional which maps any sequence $A$ to a sequence $B$, such that given any restriction of $B$ to a set of positive upper density, one can recover $A$. This implies that if $A$ is non-computable, then $B$ is absolutely undecidable. Using a forcing construction, we show that this result cannot be strengthened in any significant fashion.

We examine a definition of the mutual information of two reals proposed by Levin in [5]. The mutual information is $$I(A:B)=\log\sum\limits_{\sigma,\tau\in 2^{<\omega}} 2^{K(\sigma)-K^A(\sigma)+K(\tau)-K^B(\tau)-K(\sigma,\tau)},$$ where $K(\cdot)$ is the prefix-free Kolmogorov complexity. A real $A$ is said to have finite self-information if $I(A:A)$ is finite. We give a construction for a perfect $\Pi^0_1$ class of reals with this property, which settles some open questions posed by Hirschfeldt and Weber. The construction produces a perfect set of reals with $K(\sigma)\leq^+ K^{A}(\sigma)+f(\sigma)$ for any given $\Delta^0_2$ $f$ with a particularly nice approximation and for a specific choice of $f$ it can also be used to produce a perfect $\Pi^0_1$ set of reals that are low for effective Hausdorff dimension and effective packing dimension. The construction can be further adapted to produce a single perfect set of reals that satisfy $K(\sigma) \leq^+ K^A(\sigma)+f(\sigma)$ for all $f$ in a ‘nice' class of $\Delta^0_2$ functions which includes all $\Delta^0_2$ orders.

S-measures are Loeb measures restricted to the sigma algebra generated by standard sets. This paper gives new extensions of the S-measure machinery, with applications to standard measure theory.

We give a full description of the structure under inclusion of all finite level Borel classes of functions, and provide an elementary proof of the well-known fact that not every Borel function can be written as a countable union of $\boldsymbol{\Sigma}^0_\alpha$-measurable functions (for every fixed $1 \leq \alpha < \omega_1$). Moreover, we present some results concerning those Borel functions which are $\omega$-decomposable into continuous functions (also called countably continuous functions in the literature): such results should be viewed as a contribution towards the goal of generalizing a remarkable theorem of Jayne and Rogers to all finite levels, and in fact they allow us to prove some restricted forms of such generalizations. We also analyze finite level Borel functions in terms of composition of simpler functions, and we finally present an application to Banach space theory.

A homomorphism from a completely metrizable topological group into a free product of groups whose image is not contained in a factor of the free product is shown to be continuous with respect to the discrete topology on the range. In particular, any completely metrizable group topology on a free product is discrete.

In contrast with the notion of complexity, a set $A$ is called anti-complex if the Kolmogorov complexity of the initial segments of $A$ chosen by a recursive function is always bounded by the identity function. We show that, as for complexity, the natural arena for examining anti-complexity is the weak-truth table degrees. In this context, we show the equivalence of anti-complexity and other lowness notions such as r.e. traceability or being weak truth-table reducible to a Schnorr trivial set. A set $A$ is anti-complex if and only if it is reducible to another set $B$ with tiny use, whereby we mean that the use function for reducing $A$ to $B$ can be made to grow arbitrarily slowly, as gauged by unbounded nondecreasing recursive functions. This notion of reducibility is then studied in its own right, and we also investigate its range and the range of its uniform counterpart.

The following question is open: Does there exist a hyperarithmetic class of computable structures with exactly one non-hyperarithmetic isomorphism-type? Given any oracle $a \in 2^\omega$, we can ask the same question relativized to $a$. A negative answer for every $a$ implies Vaught's Conjecture for $L_{\omega_1 \omega}$.