The Schroder-Bernstein property for a-saturated models

A first-order theory T has the Schr\"oder-Bernstein (SB) property if any pair of elementarily bi-embeddable models are isomorphic. We prove that T has an expansion by constants that has the SB property if and only if T is superstable and non-multidimensional. We also prove that among superstable theories T, the class of a-saturated models of T has the SB property if and only if T has no nomadic types.Comment: 13 page

Pseudosaturation and the Interpretability Orders

We streamline treatments of the interpretability orders $\trianglelefteq^*_\kappa$ of Shelah, the key new notion being that of pseudosaturation. Extending work of Malliaris and Shelah, we classify the interpretability orders on the stable theories. As a further application, we prove that for all countable theories $T_0, T_1$, if $T_1$ is unsupersimple, then $T_0 \trianglelefteq^*_1 T_1$ if and only if $T_0 \trianglelefteq^*_{\aleph_1} T_1$. We thus deduce that simplicity is a dividing line in $\trianglelefteq^*_{\aleph_1}$, and that consistently, $SOP_2$ characterizes maximality in $\trianglelefteq^*_{\aleph_1}$; previously these results were only known for $\trianglelefteq^*_1$.Comment: 42 page

Henkin constructions of models with size continuum

We survey the technique of constructing customized models of size continuum in omega steps and illustrate the method by giving new proofs of mostly old results within this rubric. One new theorem, which is joint with Saharon Shelah, is that a pseudominimal theory has an atomic model of size continuum

Independence relations in randomizations

The randomization of a complete first order theory $T$ is the complete continuous theory $T^R$ with two sorts, a sort for random elements of models of $T$, and a sort for events in an underlying probability space. We study various notions of independence in models of $T^R$.Comment: 45 page

The uncountable spectra of countable theories

Let T be a complete, first-order theory in a finite or countable language having infinite models. Let I(T,kappa) be the number of isomorphism types of models of T of cardinality \kappa. We denote by \mu (respectively \hat\mu) the number of cardinals (respectively infinite cardinals) less than or equal to \kappa. We prove that I(T,\kappa), as a function of \kappa > \aleph_0, is the minimum of 2^{\kappa} and one of the following functions: 1. 2^{\kappa}; 2. the constant function 1; 3. |\hat\mu^n/{\sim_G}|-|(\hat\mu - 1)^n/{\sim_G}| if \hat\mu<\omega for some 1= \omega some group G <= Sym(n); 4. the constant function \beth_2; 5. \beth_{d+1}(\mu) for some infinite, countable ordinal d; 6. \sum_{i=1}^d \Gamma(i) where d is an integer greater than 0 (the depth of T) and \Gamma(i) is either \beth_{d-i-1}(\mu^{\hat\mu}) or \beth_{d-i}(\mu^{\sigma(i)} + \alpha(i)), where \sigma(i) is either 1, \aleph_0 or \beth_1, and \alpha(i) is 0 or \beth_2; the first possibility for \Gamma(i) can occur only when d-i > 0.Comment: 51 pages, published version, abstract added in migratio

Semi-isolation and the strict order property

We study semi-isolation as a binary relation on the locus of a complete type and prove that under some additional assumptions it induces the strict order property.Comment: Submitted to Notre Dame Journal of Formal Logi

A multiverse perspective on the axiom of constructiblity

I shall argue that the commonly held V not equal L via maximize position, which rejects the axiom of constructibility V = L on the basis that it is restrictive, implicitly takes a stand in the pluralist debate in the philosophy of set theory by presuming an absolute background concept of ordinal. The argument appears to lose its force, in contrast, on an upwardly extensible concept of set, in light of the various facts showing that models of set theory generally have extensions to models of V = L inside larger set-theoretic universes.Comment: 21 pages. This article expands on an argument that I made during my talk at the Asian Initiative for Infinity: Workshop on Infinity and Truth, held July 25--29, 2011 at the Institute for Mathematical Sciences, National University of Singapore. Commentary concerning this paper can be made at http://jdh.hamkins.org/multiverse-perspective-on-constructibilit

On uncountable hypersimple unidimensional theories

We extend a dichotomy between 1-basedness and supersimplicity proved in a previous paper. The generalization we get is to arbitrary language, with no restrictions on the topology (we do not demand type-definabilty of the open set in the definition of essential 1-basedness). We conclude that every (possibly uncountable) hypersimple unidimensional theory that is not s-essentially 1-based by means of the forking topology is supersimple. We also obtain a strong version of the above dichotomy in the case where the language is countable

Properly ergodic structures

We consider ergodic $\mathrm{Sym}(\mathbb{N})$-invariant probability measures on the space of $L$-structures with domain $\mathbb{N}$ (for $L$ a countable relational language), and call such a measure a properly ergodic structure when no isomorphism class of structures is assigned measure $1$. We characterize those theories in countable fragments of $\mathcal{L}_{\omega_1, \omega}$ for which there is a properly ergodic structure concentrated on the models of the theory. We show that for a countable fragment $F$ of $\mathcal{L}_{\omega_1, \omega}$ the almost-sure $F$-theory of a properly ergodic structure has continuum-many models (an analogue of Vaught's Conjecture in this context), but its full almost-sure $\mathcal{L}_{\omega_1, \omega}$-theory has no models. We also show that, for an $F$-theory $T$, if there is some properly ergodic structure that concentrates on the class of models of $T$, then there are continuum-many such properly ergodic structures.Comment: 41 page

Generalized amalgamation and homogeneity

In this paper we shall prove that any $2$-transitive finitely homogeneous structure with a supersimple theory satisfying a generalized amalgamation property is a random structure. In particular, this adapts a result of Koponen for binary homogeneous structures to arbitrary ones without binary relations. Furthermore, we point out a relation between generalized amalgamation, triviality and quantifier elimination in simple theories