### A rigid Boolean algebra that admits the elimination of Q21

Using ♢ , we construct a rigid atomless Boolean algebra that has no uncountable antichain and that admits the elimination of the Malitz quantifier ${Q}_{1}^{2}$.

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Using ♢ , we construct a rigid atomless Boolean algebra that has no uncountable antichain and that admits the elimination of the Malitz quantifier ${Q}_{1}^{2}$.

For a cardinal μ we give a sufficient condition ${\oplus}_{\mu}$ (involving ranks measuring existence of independent sets) for: ${\otimes}_{\mu}$ if a Borel set B ⊆ ℝ × ℝ contains a μ-square (i.e. a set of the form A × A with |A| =μ) then it contains a ${2}^{{\aleph}_{0}}$-square and even a perfect square, and also for ${\otimes}_{\mu}^{\text{'}}$ if $\psi \in {L}_{{\omega}_{1},\omega}$ has a model of cardinality μ then it has a model of cardinality continuum generated in a “nice”, “absolute” way. Assuming $MA+{2}^{{\aleph}_{0}}>\mu $ for transparency, those three conditions (${\oplus}_{\mu}$, ${\otimes}_{\mu}$ and ${\otimes}_{\mu}^{\text{'}}$) are equivalent, and from this we deduce that...

We lower substantially the strength of the assumptions needed for the validity of certain results in category theory and homotopy theory which were known to follow from Vopěnka’s principle. We prove that the necessary large-cardinal hypotheses depend on the complexity of the formulas defining the given classes, in the sense of the Lévy hierarchy. For example, the statement that, for a class $\mathcal{S}$ of morphisms in a locally presentable category $\mathcal{C}$ of structures, the orthogonal class of objects is a small-orthogonality...

A 1984 problem of S. Z. Ditor asks whether there exists a lattice of cardinality ℵ₂, with zero, in which every principal ideal is finite and every element has at most three lower covers. We prove that the existence of such a lattice follows from either one of two axioms that are known to be independent of ZFC, namely (1) Martin’s Axiom restricted to collections of ℵ₁ dense subsets in posets of precaliber ℵ₁, (2) the existence of a gap-1 morass. In particular, the existence of such a lattice is consistent...

By results of [9] there are models and for which the Ehrenfeucht-Fraïssé game of length ω₁, $EF{G}_{\omega \u2081}(,)$, is non-determined, but it is consistent relative to the consistency of a measurable cardinal that no such models have cardinality ≤ ℵ₂. We now improve the work of [9] in two ways. Firstly, we prove that the consistency strength of the statement “CH and $EF{G}_{\omega \u2081}(,)$ is determined for all models and of cardinality ℵ₂” is that of a weakly compact cardinal. On the other hand, we show that if ${2}^{\aleph \u2080}<{2}^{\aleph \u2083}$, T is a countable complete...

We study the Borel reducibility of Borel equivalence relations on the generalized Baire space ${\kappa}^{\kappa}$ for an uncountable κ with ${\kappa}^{<\kappa}=\kappa $. The theory looks quite different from its classical counterpart where κ = ω, although some basic theorems do generalize.

Let κ be an uncountable regular cardinal. Call an equivalence relation on functions from κ into 2 second order definable over H(κ) if there exists a second order sentence ϕ and a parameter P ⊆ H(κ) such that functions f and g from κ into 2 are equivalent iff the structure ⟨ H(κ), ∈, P, f, g ⟩ satisfies ϕ. The possible numbers of equivalence classes of second order definable equivalence relations include all the nonzero cardinals at most κ⁺. Additionally, the possibilities are closed under unions...

Let λ be an infinite cardinal number. The ordinal number δ(λ) is the least ordinal γ such that if ϕ is any sentence of ${L}_{\lambda \u207a\omega}$, with a unary predicate D and a binary predicate ≺, and ϕ has a model ℳ with $\u27e8{D}^{\mathcal{M}},{\prec}^{\mathcal{M}}\u27e9$ a well-ordering of type ≥ γ, then ϕ has a model ℳ ’ where $\u27e8{D}^{{\mathcal{M}}^{\text{'}}},{\prec}^{{\mathcal{M}}^{\text{'}}}\u27e9$ is non-well-ordered. One of the interesting properties of this number is that the Hanf number of ${L}_{\lambda \u207a\omega}$ is exactly ${\beth}_{\delta \left(\lambda \right)}$. It was proved in [BK71] that if ℵ₀ < λ < κ$areregularcardinalnumbers,thenthereisaforcingextension,preservingcofinalities,suchthatintheextension$2λ = κ$and\delta \left(\lambda \right)<\lambda \u207a\u207a.Weimprovethisresultbyprovingthefollowing:Suppose\aleph \u2080<\lambda <\theta \le \kappa arecardinalnumberssuchthat$∙ ${\lambda}^{<\lambda}=\lambda $; ∙ cf(θ) ≥ λ⁺ and ${\mu}^{\lambda}<\theta $ whenever μ < θ; ∙ ${\kappa}^{\lambda}=\kappa $. Then there is a forcing...

We study a notion of potential isomorphism, where two structures are said to be potentially isomorphic if they are isomorphic in some generic extension that preserves stationary sets and does not add new sets of cardinality less than the cardinality of the models. We introduce the notion of weakly semi-proper trees, and note that there is a strong connection between the existence of potentially isomorphic models for a given complete theory and the existence of weakly semi-proper trees. ...

It is a well-known fact that modules over a commutative ring in general cannot be classified, and it is also well-known that we have to impose severe restrictions on either the ring or on the class of modules to solve this problem. One of the restrictions on the modules comes from freeness assumptions which have been intensively studied in recent decades. Two interesting, distinct but typical examples are the papers by Blass [1] and Eklof [8], both jointly with Shelah. In the first case the authors...

It is shown that for every Darboux function F there is a non-constant continuous function f such that F + f is still Darboux. It is shown to be consistent - the model used is iterated Sacks forcing - that for every Darboux function F there is a nowhere constant continuous function f such that F + f is still Darboux. This answers questions raised in [5] where it is shown that in various models of set theory there are universally bad Darboux functions, Darboux functions whose sum with any nowhere...

For a cardinal κ and a model M of cardinality κ let No(M) denote the number of nonisomorphic models of cardinality κ which are ${L}_{\infty ,\kappa}$-equivalent to M. We prove that for κ a weakly compact cardinal, the question of the possible values of No(M) for models M of cardinality κ is equivalent to the question of the possible numbers of equivalence classes of equivalence relations which are Σ¹₁-definable over ${V}_{\kappa}$. By [SV] it is possible to have a generic extension where the possible numbers of equivalence classes...

We sort out to a large extent when a (first order complete theory) T has a superlimit model in a cardinal λ. Also we deal with related notions of being limit.