On uncountable abelian groups

We continue the investigation from [10], [11], [12] on uncountable abelian groups. This paper tends more to group theory and was motivated by Nunke’s statement (in [9]) that Whitehead problem, rephrased properly, is not solved yet. The author would like to thank the NSF for partially supporting this research by grants 144-H747 and M2S76-08479, and the BSF for partially supporting this research by grant 1110.     

Universal Graphs with Forbidden Subgraphs and Algebraic Closure

We apply model theoretic methods to the problem of existence of countable universal graphs with finitely many forbidden connected subgraphs. We show that to a large extent the question reduces to one of local finiteness of an associated “algebraic closure” operator. The main applications are new examples of universal graphs with forbidden subgraphs and simplified treatments of some previously known cases.     

A Banach space with few operators

Assuming the axiom (of set theory)V=L (explained below), we construct a Banach space with density character ℵ₁ such that every (linear bounded) operatorT fromB toB has the forma I+T ₁, whereI is the identity, andT ₁ has a separable range. The axiomV=L means that all the sets in the universe are in the classL of sets constructible from ordinals; in a sense this is the minimal universe. In fact, we make use of just one consequence of this axiom, ℵ₁ proved by Jensen, which is widely used by mathematical logicians.     

Model-theoretic applications of cofinality spectrum problems

We apply the recently developed technology of cofinality spectrum problems to prove a range of theorems in model theory. First, we prove that any model of Peano arithmetic is λ-saturated iff it has cofinality ≥ λ and the underlying order has no (κ, κ)-gaps for regular κ < λ. We also answer a question about balanced pairs of models of PA. Second, assuming instances of GCH, we prove that SOP ₂ characterizes maximality in the interpretability order ?*, settling a prior conjecture and proving that SOP ₂ is a real dividing line. Third, we establish the beginnings of a structure theory for NSOP ₂, proving that NSOP ₂ can be characterized by the existence of few so-called higher formulas. In the course of the paper, we show that p_s = t_s in any weak cofinality spectrum problem closed under exponentiation (naturally defined). We also prove that the local versions of these cardinals need not coincide, even in cofinality spectrum problems arising from Peano arithmetic.     

Absolute E-rings

A ring R with 1 is called an E-ring if EndZR is ring-isomorphic to R under the canonical homomorphism taking the value 1σ for any σ∈EndZR. Moreover R is an absolute E-ring if it remains an E-ring in every generic extension of the universe. E-rings are an important tool for algebraic topology as explained in the introduction. The existence of an E-ring R of each cardinality of the form λℵ₀ was shown by Dugas, Mader and Vinsonhaler (1987) [9]. We want to show the existence of absolute E-rings. It turns out that there is a precise cardinal-barrier κ(ω) for this problem: (The cardinal κ(ω) is the first ω-Erd?s cardinal defined in the introduction. It is a relative of measurable cardinals.) We will construct absolute E-rings of any size λ<κ(ω). But there are no absolute E-rings of cardinality ?κ(ω). The non-existence of huge, absolute E-rings ?κ(ω) follows from a recent paper by Herden and Shelah (2009) [24] and the construction of absolute E-rings R is based on an old result by Shelah (1982) [31] where families of absolute, rigid colored trees (with no automorphism between any distinct members) are constructed. We plant these trees into our potential E-rings with the aim to prevent unwanted endomorphisms of their additive group to survive. Endomorphisms will recognize the trees which will have branches infinitely often divisible by primes. Our main result provides the existence of absolute E-rings for all infinite cardinals λ<κ(ω), i.e. these E-rings remain E-rings in all generic extensions of the universe (e.g. using forcing arguments). Indeed all previously known E-rings (Dugas, Mader and Vinsonhaler, 1987 [9]; Göbel and Trlifaj, 2006 [23]) of cardinality ?ℵ₀² have a free additive group R⁺ in some extended universe, thus are no longer E-rings, as explained in the introduction. Our construction also fills all cardinal-gaps of the earlier constructions (which have only sizes λℵ₀). These E-rings are domains and as a by-product we obtain the existence of absolutely indecomposable abelian groups, compare Göbel and Shelah (2007) [22].     

More constructions for Boolean algebras

 We construct Boolean algebras with prescribed behaviour concerning depth for the free product of two Boolean algebras over a third, in ZFC using pcf; assuming squares we get results on ultraproducts. We also deal with the family of cardinalities and topological density of homomorphic images of Boolean algebras (you can translate it to topology - on the cardinalities of closed subspaces); and lastly we deal with inequalities between cardinal invariants, mainly . Received: 9 September 1998 / Published online: 7 May 2002     

Two cardinals models with gap one revisited

We succeed to say something on the identities of (μ⁺, μ) when μ > θ > cf(μ) with μ strong limit θ‐compact or even μ is limit of compact cardinals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Middle diamond

Under certain cardinal arithmetic assumptions, we prove that for every large enough regular λ cardinal, for many regular κ < λ, many stationary subsets of λ concentrating on cofinality κ has the “middle diamond”. In particular, we have the middle diamond on {δ < λ: cf(δ) = κ}. This is a strong negation of uniformization.I would like to thank Alice Leonhardt for the beautiful typing. This research was partially supported by the Israel Science Foundation. Publication 775.     

On two problems of Erdos and Hechler: New methods in singular madness

For an infinite cardinal , denotes the set of all cardinalities of nontrivial maximal almost disjoint families over .

Erdos and Hechler proved in 1973 the consistency of for a singular cardinal and asked if it was ever possible for a singular that , and also whether for every singular cardinal .

We introduce a new method for controlling for a singular and, among other new results about the structure of for singular , settle both problems affirmatively.