Almost disjoint families of connected sets

We investigate the question which (separable metrizable) spaces have a ‘large’ almost disjoint family of connected (and locally connected) sets. Every compact space of dimension at least 2 as well as all compact spaces containing an ‘uncountable star’ have such a family. Our results show that the situation for 1-dimensional compacta is unclear.     

The spectrum of partitions of a Boolean algebra

The main notion dealt with in this article is

Some characterizations of spaces that have transfinite dimension

We characterize separable metrizable spaces that have small transfinite dimension and metrizable spaces that have large transfinite dimension modifying two classical characterizations of countable-dimensional spaces and applying the notion of a strongly point-finite family.     

Homogeneous almost disjoint families

An almost disjoint family is constructed which is isomorphic to any almost disjoint family which can be constructed from it by taking subsets and finite unions. This is applied to the construction of a Boolean algebra with related properties.Presented by R. McKenzie.Partially supported by Israel-US Binational Research Fund.Partially supported by NSERC.     

A short proof of existence of disjoint hypercyclic operators

We give a short proof of existence of disjoint hypercyclic tuples of operators of any given length on any separable infinite dimensional Fréchet space. Similar argument provides disjoint dual hypercyclic tuples of operators of any length on any infinite dimensional Banach space with separable dual.     

Martin’s Axiom and separated mad families

Two families \(\mathcal{A}, \mathcal{B}\) of subsets of ω are said to be separated if there is a subset of ω which mod finite contains every member of \(\mathcal{A}\) and is almost disjoint from every member of \(\mathcal{B}\). If \(\mathcal{A}\) and \(\mathcal{B}\) are countable disjoint subsets of an almost disjoint family, then they are separated. Luzin gaps are well-known examples of ω ₁-sized subfamilies of an almost disjoint family which can not be separated. An almost disjoint family will be said to be ω ₁-separated if any disjoint pair of ≤ω ₁-sized subsets are separated. It is known that the proper forcing axiom (PFA) implies that no maximal almost disjoint family is ≤ω ₁-separated. We prove that this does not follow from Martin’s Axiom.     

Cutting Glass

Urrutia asked the following question: Given a family of pairwise disjoint compact convex sets on a sheet of glass, is it true that one can always separate from one another a constant fraction of them using edge-to-edge straight-line cuts? We answer this question in the negative, and establish some lower and upper bounds for the number of separable sets. We also consider the special cases when the family consists of intervals, axis-parallel rectangles, ``fat' sets, or ``fat' sets with bounded size. Received April 7, 1999. Online publication May 16, 2000.     

Isotype knice subgroups of global Warfield groups

If H is an isotype knice subgroup of a global Warfield group G, we introduce the notion of a k-subgroup to obtain various necessary and sufficient conditions on the quotient group G/H in order for H itself to be a global Warfield group. Our main theorem is that H is a global Warfield group if and only if G/H possesses an H(ℵ₀)-family of almost strongly separable k-subgroups. By an H(ℵ₀)-family we mean an Axiom 3 family in the strong sense of P. Hill. As a corollary to the main theorem, we are able to characterize those global k-groups of sequentially pure projective dimension ⩽ 1.     

Multiplicative sets, left strongly prime ideals, and left strongly prime radicals in rings

Left commutative multiplicative sets {ie397-01} for an associative ring R are defined. In particular, this notion includes commutative multiplicative sets of the associative ring. We also define the notion of a left {ie397-02}-ideal and prove that each left {ie397-03}-ideal, maximal with respect to being disjoint from {ie397-04}, is left strongly prime. Using a technique developed for insulators for a left ideal, we also characterize the left strongly prime radical of a left ideal of the ring R, which was known only for two-sided ideals.     

Strongly extreme points in Banach spaces

In this paper some properties of a special type of boundary point of convex sets in Banach spaces are studied. Specifically, a strongly extreme point x of a convex set S is a point of S such that for each real number r>0, segments of length 2r and centered x are not uniformly closer to S than some positive number d(x,r). Results are obtained comparing the notion of strongly extreme point to other known types of special boundary points of convex sets. Using the notion of strongly extreme point, a convexity condition is defined on the norm of the space under consideration, and this convexity condition makes possible a unified treatment of some previously studied convexity conditions. In addition, a sufficient condition is given on the norm of a separable conjugate space for every extreme point of the unit ball to be strongly extreme.     

Strongly almost disjoint functions

Two total functionsf, g fromω ₁ toω are called strongly almost disjoint if {α<ω ₁:f(α)=g(α)} is finite. We show that it is consistent with ZFC to have families of pairwise strongly almost disjoint functions of arbitrary prescribed size.     

Hedonic coalition formation games: A new stability notion

This paper studies hedonic coalition formation games where each player’s preferences rely only upon the members of her coalition. A new stability notion under free exit-free entry membership rights, referred to as strong Nash stability, is introduced which is stronger than both core and Nash stabilities studied earlier in the literature. Strong Nash stability has an analogue in non-cooperative games and it is the strongest stability notion appropriate to the context of hedonic coalition formation games. The weak top-choice property is introduced and shown to be sufficient for the existence of a strongly Nash stable partition. It is also shown that descending separable preferences guarantee the existence of a strongly Nash stable partition. Strong Nash stability under different membership rights is also studied.     

Real Taylor shifts and simultaneous approximation

We are interested in the Taylor shift operator acting on the space of infinitely differentiable functions. In particular if we choose a countable family of centers of Taylor expansion, we prove that the associated family of real Taylor shifts fulfills the approximation of any given family of infinitely differentiable functions with a common subsequence of iterates applied on a common vector. We obtain similar conclusions in the context of universal series improving recent statements. Finally we introduce the notion of doubly universal Taylor shift. All these results give new and natural examples of disjoint universality.     

On Borel almost disjoint families

There is a close correspondence between uncountable almost disjoint families of subsets of $\omega $ and Aleksandrov–Urysohn compacta (in short, AU-compacta)—separable, uncountable compact spaces whose second derived set is a singleton. We shall show in particular, that AU-compacta embeddable in the space of first Baire class functions on the Cantor set $2^\omega $ , with the pointwise topology, are exactly the ones determined by almost disjoint families that are Borel sets in $2^\omega $ , and they are also distinguished among AU-compacta by the property that the cylindrical $\sigma $ -algebras of their function spaces are standard measurable spaces. Although the first condition implies the third one for arbitrary separable compact space, it is an open problem, whether the reverse implication is always true.     

A sufficient condition for strong almost-periodicity of scalarly almost periodic representations of the group of real numbers

We prove the following theorem: If every separable subspace Y of a Banach space X has a separable weak sequential closure in Y ^**, then every scalarly almost periodic group acting in X is strongly almost periodic. Kharkov State Academy of Municipal Economy, Kharkov. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 49, No. 4, pp. 523–526, April, 1997.     

Existence and nonexistence of hypercyclic semigroups

In these notes we provide a new proof of the existence of a hypercyclic uniformly continuous semigroup of operators on any separable infinite-dimensional Banach space that is very different from--and considerably shorter than--the one recently given by Bermúdez, Bonilla and Martinón. We also show the existence of a strongly dense family of topologically mixing operators on every separable infinite-dimensional Fréchet space. This complements recent results due to Bès and Chan. Moreover, we discuss the Hypercyclicity Criterion for semigroups and we give an example of a separable infinite-dimensional locally convex space which supports no supercyclic strongly continuous semigroup of operators.

     

Generation of disjointly constrained bilinear programming test problems

This paper describes a technique for generating disjointly constrained bilinear programming test problems with known solutions and properties. The proposed construction technique applies a simple random transformation of variables to a separable bilinear programming problem that is constructed by combining disjoint low-dimensional bilinear programs.     

Disjointness in hypercyclicity

We introduce a notion of disjointness for finitely many hypercyclic operators acting on a common space, notion that is weaker than Furstenberg's disjointness of fluid flows. We provide a criterion to construct disjoint hypercyclic operators, that generalizes some well-known connections between the Hypercyclicity Criterion, hereditary hypercyclicity and topological mixing to the setting of disjointness in hypercyclicity. We provide examples of disjoint hypercyclic operators for powers of weighted shifts on a Hilbert space and for differentiation operators on the space of entire functions on the complex plane.     

On the Shape of Compact Hypersurfaces with Almost‐Constant Mean Curvature

The distance of an almost‐constant mean curvature boundary from a finite family of disjoint tangent balls with equal radii is quantitatively controlled in terms of the oscillation of the scalar mean curvature. This result allows one to quantitatively describe the geometry of volume‐constrained stationary sets in capillarity problems.© 2017 Wiley Periodicals, Inc.     

Large sets with holes

We study large sets of disjoint Steiner triple systems “with holes”. The purpose of these structures is to extend the use of large sets of disjoint Steiner triple systems in the construction of various other large set type structures to values of v for which no Steiner triple system of order v exists, i.e., v ≡ 0, 2, 4, or 5 (mod 6). We give constructions for all of these congruence classes. We describe several applications, including applications to large sets of disjoint group divisible designs and to large sets of disjoint separable ordered designs. © 1993 John Wiley & Sons, Inc.