Countable ultraproducts without CH

An important application of ultrafilters is in the ultraproduct construction in model theory. In this paper we study ultraproducts of countable structures, whose universe we assume is ω, using ultrafilters on a countable index set, which we also assume to be ω. Many of the properties of the ultraproduct are in fact inherent properties of the ultrafilter. For example, if we take a sequence of countable linear orders without maximal element, then their ultraproduct will have no maximal element, and we can ask what its cofinality is. This cardinal depends only on the ultrafilter; it does not depend on what linear orders comprise the factors.     

Countable models of ℵ1-categorical theories

Countable models of ℵ₁-categorical theories are classified. It is shown that such a theory has only a countable number of nonisomorphic countable models. The author was partially supported by NSF grants GP-1621 and GP-4257 during the period those results were obtained.     

On Theories Having Three Countable Models

A theory T is called almost ??₀-categorical if for any pure types p₁(x₁),…,p_n(x_n) there are only finitely many pure types which extend p₁(x₁) ∪…∪p_n(x_n). It is shown that if T is an almost ??₀-categorical theory with I(??₀,T) = 3, then a dense linear ordering is interpretable in T.     

Countable infinite existentially closed models of universally axiomatizable theories

In the present article, we obtain a new criterion for amodel of a universally axiomatizable theory to be existentially closed. The notion of a maximal existential type is used in the proof and for investigating properties of countable infinite existentially closed structures. The notions of a prime and a homogeneous model, which are classical for the general model theory, are introduced for such structures. We study universal theories with the joint embedding property admitting a single countable infinite existentially closed model. We also construct, for every natural n, an example of a complete inductive theory with a countable infinite family of countable infinite models such that n of them are existentially closed and exactly two are homogeneous.     

The Existence of the Attractor of Countable Iterated Function Systems

The theory of iterated function systems (IFS) and of infinite iterated function systems consisting of contraction mappings has been studied in the last decades. Some extensions of the spaces and the contractions concern many authors in fractal theory. In this paper there are described some results in that topic concerning the existence and uniqueness of nonempty compact set which is a set ”fixed point” of a countable iterated function system (CIFS). Moreover, some approximations of the attractor of a CIFS by the attractors of the partial IFSs are given.     

Entropic Moments and Domains of Attraction on Countable Alphabets

Modern information theory is largely developed in connection with random elements residing in large, complex, and discrete data spaces, or alphabets. Lacking natural metrization and hence moments, the associated probability and statistics theory must rely on information measures in the form of various entropies, for example, Shannon’s entropy, mutual information and Kullback–Leibler divergence, which are functions of an entropic basis in the form of a sequence of entropic moments of varying order. The entropicmoments collectively characterize the underlying probability distribution on the alphabet, and hence provide an opportunity to develop statistical procedures for their estimation. As such statistical development becomes an increasingly important line of research in modern data science, the relationship between the underlying distribution and the asymptotic behavior of the entropic moments, as the order increases, becomes a technical issue of fundamental importance. This paper offers a general methodology to capture the relationship between the rates of divergence of the entropic moments and the types of underlying distributions, for a special class of distributions. As an application of the established results, it is demonstrated that the asymptotic normality of the remarkable Turing’s formula for missing probabilities holds under distributions with much thinner tails than those previously known.     

The Ascending Tree Condition: Constructive Algebra Without Countable Choice

Abstract

A strengthening of the ascending chain condition allows a choice-free constructive development of the theory of Noetherian modules. Related topics in the theory of PID's and elementary divisor rings are also explored.     

Spaces in Which Countable Closed Sets Have Countable Character

It is proved that in a T ₃ space countable closed sets have countable character if and only if the set of limit point of the space is a countable compact set and every compact set is of countable character. Also, it is shown that spaces where countable sets have countable character are WN-spaces and are very close to M-spaces. Finally, some questions of Dai and Lia are discussed and some questions are proposed.     

A Model-Theoretic Characterization of Countable Direct Sums of Finite Cyclic Groups #

We prove that an abelian group G is a countable direct sum of finite cyclic groups if and only if there exists a consistent existential theory Γ of abelian groups such that G is embeddable in every model of Γ.     

Embedding of Countable Topological Semigroups in Simple Countable Connected Topological Semigroups

We prove that any countable Hausdorff topological (inverse) semigroup is topologically isomorphically embedded into a simple countable connected Hausdorff topological (inverse) semigroup with identity.     

Linear Operators Generated by a Countable Number of Quasi-differential Expressions

The theory of linear ordinary quasi-differential operators has been considered in Lebesgue locally integrable spaces on a single interval of the real line. Such spaces are not Banach spaces but can be considered as complete, locally convex, linear topological spaces where the topology is derived from a countable family of semi-norms. The first conjugate space can also be defined as a complete, locally convex, linear topological space but now with the topology derived as a strict inductive limit. This article extends the previous single interval results to the case when a finite or countable number of intervals of the real line is considered. Conjugate and preconjugate linear quasi-differential operators are defined and relationships between these operators are developed.     

A Classification of Countable Lower 1-transitive Linear Orders

This paper contains a classification of countable lower 1-transitive linear orders. This is the first step in the classification of countable 1-transitive trees given in Chicot and Truss (2009): the notion of lower 1-transitivity generalises that of 1-transitivity for linear orders, and it is essential for the structure theory of 1-transitive trees. The classification is given in terms of coding trees, which describe how a linear order is fabricated from simpler pieces using concatenations, lexicographic products and other kinds of construction. We define coding trees and show that a coding tree can be constructed from a lower 1-transitive linear order \((X, \leqslant )\) by examining all the invariant partitions on X. Then we show that a lower 1-transitive linear order can be recovered from a coding tree up to isomorphism.     

On Countable Products of Finite Hausdorff Spaces

We investigate in ZF (i.e., Zermelo‐Fraenke set theory without the axiom of choice) conditions that are necessary and sufficient for countable products ∏_m∈ℕX_m of (a) finite Hausdorff spaces X_m resp. (b) Hausdorff spaces X_m with at most n points to be compact resp. Baire. Typica results: (i) Countable products of finite Hausdorff spaces are compact (resp. Baire) if and only if countable products of non‐empty finite sets are non‐empty. (ii) Countable products of discrete spaces with at most n + 1 points are compact (resp. Baire) if and only if countable products of non‐empty sets with at most n points are non‐empty.     

可列集合套

[1]中给出模糊集的分解定理与表现定理，指出可利用集合套H＝{H（λ）|λ∈[0,1]}刻化模糊集。本文定义可列集合套H＝{H（a）|a∈Q}，Q是（0，1）的可列稠密子集。相应地给出新的分解定理与表现定理。指出可利用可列集合套刻化的模糊集。     

亚紧空间的可数乘积

In this paper,we present that if Y is a hereditarily metacompact space and{Xn:n∈ω}is a countable collection of Cech-scattered metacompact spaces,then the followings are∏equivalent:(1)Y×∏n∈ωXn is metacompact,(2)Y×∏n∈ωXn is countable metacompact,(3)Y×n∈ωXn is orthocompact.Thereby,this result generalizes Theorem 5.4 in[Tanaka,Tsukuba.J.Math.,1993,17:565–587].In addition,we obtain that if Y is a hereditarilyσ-metacompact space and{Xn:n∈ω∏}is a countable collection of Cech-scatteredσ-metacompact spaces,then the product Y×n∈ωXn isσ-metacompact.     

Countable categoricity

Translated from Algebra i Logika, Vol. 27, No. 6, pp. 711–714, November–December, 1988.     

Countable sequenceable groups

A finite group G having n elements is said to be sequenceable if there exists an arrangement b₁, b₂…,b_n of its elements such that b₁, b₁b₂,…, b₁b₂? b₁ is also an arrangement of the elements of G. This definition can be extended to the case when G is countably infinite by requiring the existence of a sequence b₁, b₂,…, containing each element of G exactly once, such that the sequence b₁,b₁b₂,… also contains each element of G exactly once. With this definition every countably-infinite group is sequenceable.     

Countable partition ordinals

The structure of ordinals of the form ω^ωβ for countable β is studied. The main result is:

Theorem 1. Ifβ<ω₁is the sum of one or two indecomposable ordinals, then

ω^ωβ→(ω^ωβ,3)².     

Generating Countable Sets of Permutations

Let E be an infinite set. In answer to a question of Wagon,I show that every countable subset of the symmetric group Sym(E)is contained in a 2-generator subgroup of Sym(E). In answerto a question of Macpherson and Neumann, I show that, if Sym(E)is generated by A B where |B| ||E||, then Sym(E) is generatedby A {} for some permutation in Sym(E).     

Countable Primitive Groups

We give a complete characterization of countable primitive groups in several settings including linear groups, subgroups of mapping class groups, groups acting minimally on trees and convergence groups. The latter category includes as a special case Kleinian groups as well as subgroups of word hyperbolic groups. As an application we calculate the Frattini subgroup in many of these settings, often generalizing results that were only known for finitely generated groups. In particular, we answer a question of G. Higman and B.H. Neumann on the Frattini group of an amalgamated product. Received: January 2006, Revision: May 2006, Accepted: May 2006