Some Representation Theorem for nonreflexive Banach space ultrapowers under the Continuum Hypothesis

In this paper it will be shown that assuming the Continuum Hypothesis (CH) every nonreflexive Banach space ultrapower is isometrically isomorphic to the space of continuous, bounded and real-valued functions on the Parovicenko space. This Representation Theorem will be helpful in proving some facts from geometry and topology of nonreflexive Banach space ultrapowers.Comment: 12 page

Possible Size of an ultrapower of omega

Let omega be the first infinite ordinal (or the set of all natural numbers) with the usual order <. In section 1 we show that, assuming the consistency of a supercompact cardinal, there may exist an ultrapower of omega, whose cardinality is (1) a singular strong limit cardinal, (2) a strongly inaccessible cardinal. This answers two questions in [CK], modulo the assumption of supercompactness. In section 2 we construct several lambda-Archimedean ultrapowers of omega under some large cardinal assumptions. For example, we show that, assuming the consistency of a measurable cardinal, there may exist a lambda-Archimedean ultrapower of omega for some uncountable cardinal lambda. This answers a question in [KS], modulo the assumption of measurability

A Survey of Ultraproduct Constructions in General Topology

We survey various attempts to transport the ultraproduct construction from the realm of model theory to that of general topology

Easton's Theorem for Ramsey and Strongly Ramsey cardinals

We show that, assuming GCH, if $\kappa$ is a Ramsey or a strongly Ramsey cardinal and $F$ is a class function on the regular cardinals having a closure point at $\kappa$ and obeying the constraints of Easton's theorem, namely, $F(\alpha)\leq F(\beta)$ for $\alpha\leq\beta$ and \alpha<\cf(F(\alpha)), then there is a cofinality preserving forcing extension in which $\kappa$ remains Ramsey or strongly Ramsey respectively and $2^\delta=F(\delta)$ for every regular cardinal $\delta$.Comment: 21 page

Natural numerosities of sets of tuples

We consider a notion of "numerosity" for sets of tuples of natural numbers, that satisfies the five common notions of Euclid's Elements, so it can agree with cardinality only for finite sets. By suitably axiomatizing such a notion, we show that, contrasting to cardinal arithmetic, the natural "Cantorian" definitions of order relation and arithmetical operations provide a very good algebraic structure. In fact, numerosities can be taken as the non-negative part of a discretely ordered ring, namely the quotient of a formal power series ring modulo a suitable ("gauge") ideal. In particular, special numerosities, called "natural", can be identified with the semiring of hypernatural numbers of appropriate ultrapowers of N.Comment: 17 page

Boolean ultrapowers, the Bukovsky-Dehornoy phenomenon, and iterated ultrapowers

We show that while the length $\omega$ iterated ultrapower by a normal ultrafilter is a Boolean ultrapower by the Boolean algebra of Prikry forcing, it is consistent that no iteration of length greater than $\omega$ (of the same ultrafilter and its images) is a Boolean ultrapower. For longer iterations, where different ultrafilters are used, this is possible, though, and we give Magidor forcing and a generalization of Prikry forcing as examples. We refer to the discovery that the intersection of the finite iterates of the universe by a normal measure is the same as the generic extension of the direct limit model by the critical sequence as the Bukovsky-Dehornoy phenomenon, and we develop a sufficient criterion (the existence of a simple skeleton) for when a version of this phenomenon holds in the context of Boolean ultrapowers. Assuming that the canonical generic filter over the Boolean ultrapower model has what we call a continuous representation, we show that the Boolean model consists precisely of those members of the intersection model that have continuously and eventually uniformly represented codes

Set theory and C*-algebras

We survey the use of extra-set-theoretic hypotheses, mainly the continuum hypothesis, in the C*-algebra literature. The Calkin algebra emerges as a basic object of interest.Comment: 16 pages; to appear in Bull. Symb. Logi

Set theoretical forcing in quantum mechanics and AdS/CFT correspondence

We show unexpected connection of Set Theoretical Forcing with Quantum Mechanical lattice of projections over some separable Hilbert space. The basic ingredient of the construction is the rule of indistinguishability of Standard and some Nonstandard models of Peano Arithmetic. The ingeneric reals introduced by M. Ozawa will correspond to simultaneous measurement of incompatible observables. We also discuss some results concerning model theoretical analysis of Small Exotic Smooth Structures on topological 4-space. Forcing appears rather naturally in this context and the rule of indistinguishability is crucial again. As an unexpected application we are able to approach Maldacena Conjecture on $AdS/CFT$ correspondence in the case of AdS_5xS^5 and Super YM Conformal Field Theory in 4 dimensions. We conjecture that there is possibility of breaking Supersymetry via sources of gravity generated in 4 dimensions by exotic smooth structures on R^4 emerging in this context.Comment: 16 pages, 1 eps figure, LaTeX 2e. Presented at QCS02 held in Ustron, Poland on September 2002, to appear in Int. J. of Theor. Phy

On separably injective Banach spaces and Corrigendum to "On separably injective Banach spaces" [Adv. Math. 234 (2013) 192--216]

In this paper we deal with two weaker forms of injectivity which turn out to have a rich structure behind: separable injectivity and universal separable injectivity. We show several structural and stability properties of these classes of Banach spaces. We provide natural examples of (universally) separably injective spaces, including $\mathcal L_\infty$ ultraproducts built over countably incomplete ultrafilters, in spite of the fact that these ultraproducts are never injective. We obtain two fundamental characterizations of universally separably injective spaces: a) A Banach space $E$ is universally separably injective if and only if every separable subspace is contained in a copy of $\ell_\infty$ inside $E$. b) A Banach space $E$ is universally separably injective if and only if for every separable space $S$ one has \Ext(\ell_\infty/S, E)=0. The final Section of the paper focuses on special properties of 1-separably injective spaces. Lindenstrauss\ obtained in the middle sixties a result that can be understood as a proof that, under the continuum hypothesis, 1-separably injective spaces are 1-universally separably injective; he left open the question in {\sf ZFC}. We construct a consistent example of a Banach space of type $C(K)$ which is 1-separably injective but not 1-universally separably injective. We show that, under the continuum hypothesis, "to be universally separably injective" is not a $3$-space property, as we wrongly claimed in the paper mentioned in the title.Comment: The first paper was published in [Adv. Math. 234 (2013) 192--216]. The corrigendum is to appear in Adv. in Mat

Extremal structure in ultrapowers of Banach spaces

Given a bounded convex subset C of a Banach space X and a free ultrafilter U, we study which points (xi)U are extreme points of the ultrapower CU in XU. In general, we obtain that when { xi} is made of extreme points (respectively denting points, strongly exposed points) and they satisfy some kind of uniformity, then (xi)U is an extreme point (respectively denting point, strongly exposed point) of CU. We also show that every extreme point of CU is strongly extreme, and that every point exposed by a functional in (X*)U is strongly exposed, provided that U is a countably incomplete ultrafilter. Finally, we analyse the extremal structure of CU in the case that C is a super weakly compact or uniformly convex set. © 2022, The Author(s)