Theorem 4 is a characterization of Woodin cardinals in terms of Skolem hulls and Mostowski collapses. We define weakly hyper-Woodin cardinals and hyper-Woodin cardinals. Theorem 5 is a covering theorem for the Mitchell-Steel core model, which is constructed using total background extenders. Roughly, Theorem 5 states that this core model correctly computes successors of hyper-Woodin cardinals. W...

In this paper, we show that any finitely generated abelian-byfinite group is an elementary submodel of its profinite completion. It follows that two finitely generated abelian-by-finite groups are elementarily equivalent if and only if they have the same finite images. We give an example of two finitely generated abelian-by-finite groups G, H which satisfy these properties while G x Z and H x Z...

NSA can be introduced in multiple ways. Instead of choosing one option, these notes include three introductions. Section 1 is best-suited for those who are familiar with logic, or who want to get a flavor of model theory. Section 2 focuses on some common ingredients of various axiomatic approaches to NSA, including the star-map and the Transfer principle. Section 3 explains the ultrapower const...

After forcing which admits a very low gap—and this includes many of the forcing iterations, such as the Laver preparation, which are commonly found in the large cardinal context—every embedding j : V [G] → M [j(G)] in the extension which satisfies a mild closure condition is the lift of an embedding j : V → M in the ground model. In particular, every ultrapower embedding in the extension lifts ...

For every variety of algebras Θ and every algebra H ∈ Θ we can consider an algebraic geometry in Θ over H. Algebras in Θ may be many sorted (not necessarily one sorted) algebras. A set of sorts Γ is fixed for each Θ. This theory can be applied to the variety of representations of groups over fixed commutative ring K with unit. We consider a representation as two sorted algebra (V, G), where V i...

In the paper we prove (Theorem 8.1) that there exists a continuum of non isomorphic simple modules over KF 2 , where F 2 is a free group with 2 generators (compare with [Ca] where a continuum of non isomorphic simple 2-generated groups is constructed). Using this fact we give an example of a non action type logically Noetherian representation (Section 9). In general, the topic of this paper is ...

We prove, in ZFC, that there is an infinite strictly descending chain of classes of theories in Keisler’s order. Thus Keisler’s order is infinite and not a well order. Moreover, this chain occurs within the simple unstable theories, considered model-theoretically tame. Keisler’s order is a central notion of the model theory of the 60s and 70s which compares first-order theories (and implicitly ...

Let R be a linearly ordered ring with 1/2, Gn(R) (n ≥ 3) be the subsemigroup of GLn(R) consisting of all matrices with nonnegative elements. In [1], there is a description of all automorphisms of the semigroup Gn(R) in the case where R is a skewfield and n ≥ 2. In [2], there is a description of all automorphisms of the semigroup Gn(R) for the case where R is an arbitrary linearly ordered ring w...

There are many examples of non-isomorphic pairs of finitely generated abstract groups that are elementarily equivalent. We show that the situation in the category of profinite groups is different: If two finitely generated profinite groups are elementarily equivalent (as abstract groups), then they are isomorphic. The proof applies a result of Nikolov and Segal which in turn relies on the class...

This survey is designed as an introduction to the study of C*-algebras from the perspective of model theory for metric structures. The intended readership consists of anyone interested in learning about this subject, and naturally includes both logicians and operator algebraists. Considering this, we will not assume in these notes any previous knowledge of model theory, nor any in-depth knowled...