Pseudo definably connected definable sets

In o‐minimal structures, every cell is definably connected and every definable set is a finite union of its definably connected components. In this note, we introduce pseudo definably connected definable sets in weakly o‐minimal structures having strong cell decomposition, and prove that every strong cell in those structures is pseudo definably connected. It follows that every definable set can be written as a finite union of its pseudo definably connected components. We also show that the projections of pseudo definably connected definable sets are pseudo definably connected. Finally, we compare pseudo definable connectedness with (recently introduced) weak definable connectedness of definable sets in weakly o‐minimal structures.     

Sectionwise connected sets in vector optimization

We introduce the notion of sectionwise connected set as a new tool to investigate nonconvex vector optimization. Indeed, the image of a K-convex set through a K-quasiconnected vector function is proved to be sectionwise connected. Some properties of the minimal frontiers of sectionwise connected sets are studied.     

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.     

Minimal sets in compact connected subspaces

Let X be a topological space, f:X→X be a continuous map, and Y be a compact, connected and closed subset of X. In this paper we show that, if the boundary X_∂Y contains exactly one point v and f(v)∈Y, then Y contains a minimal set of f.     

Locally connected models for Julia sets

Let P be a polynomial with a connected Julia set J. We use continuum theory to show that it admits a finest monotone map φ onto a locally connected continuumJP_∼, i.e. a monotone map φ:J→JP_∼ such that for any other monotone map ψ:J→J^′ there exists a monotone map h with ψ=h°φ. Then we extend φ onto the complex plane C (keeping the same notation) and show that φ monotonically semiconjugates PC_| to a topological polynomialg:C→C. If P does not have Siegel or Cremer periodic points this gives an alternative proof of Kiwi's fundamental results on locally connected models of dynamics on the Julia sets, but the results hold for all polynomials with connected Julia sets. We also give a characterization and a useful sufficient condition for the map φ not to collapse all of J into a point.     

Boundedly connected sets and the distance to the intersection of two sets

We prove that if (X,d) is a metric space, C is a closed subset of X and x∈X, then the distance of x to R∩S agrees with the maximum of the distances of x to R and S, for every closed subsets R,S⊂C such that C=R∪S, if and only if C is x-boundedly connected.     

Polynomials and finite sets connected with an identity

It is shown that the polynomials satisfying the identityf(x) f(x + 1) = f(x ² +x – a), wherea either belongs to a field of characteristic zero or is transcendental over a prime field of characteristic exceeding 2, are precisely those of the form(x ² –a) ⁿ ; thus extending a result proved by Nathanson in the complex case. The result is not, in general, true in characteristic 2. Additionally, a class of finite sets, considered by Nathanson in connection with the identity, is completely determined.     

Elementary properties of arcwise connected sets and functions

The purpose of this paper is to investigate some elementary, basic properties of arcwise connected sets and functions. Since these concepts are generalizations of convexity, it is natural to ask if any of the basic properties of convex sets and functions are carried over to these new generalized classes. All the functions involved are considered to be not necessarily differentiable.     

Images of connected sets by semicontinuous multifunctions

The image of a connected set by an upper-semicontinuous (or a lower-semicontinuous) multifunction whose values are nonempty and connected is connected. We prove this theorem in its most general setting and show its usefulness in various examples from optimization and nonlinear analysis.     

On compact connected sets in Banach spaces

Let be a separable strictly convex Banach space of dimension at least 2. It is shown that there exists a nonempty compact connected set such that the nearest point mapping is not single valued on a set of points dense in . Furthermore, it is proved that most (in the sense of the Baire category) nonempty compact connected sets have the above property. Similar results hold for the furthest point mapping.

     

Omega-limit sets in hereditarily locally connected continua

In this paper we give a topological characterization of ω-limit sets in hereditarily locally connected continua. Moreover, we characterize also orbit-enclosing ω-limit sets in these spaces.     

On connected dominating sets of restricted diameter

A connected dominating set (CDS) is commonly used to model a virtual backbone of a wireless network. To bound the distance that information must travel through the network, we explicitly restrict the diameter of a CDS to be no more than s leading to the concept of a dominating s-club. We prove that for any fixed positive integer s it is NP-complete to determine if a graph has a dominating s  -club, even when the graph has diameter s+1$s+1$. As a special case it is NP-complete to determine if a graph of diameter two has a dominating clique. We then propose a compact integer programming formulation for the related minimization problem, enhance the approach with variable fixing rules and valid inequalities, and present computational results.     

Properties of connected ortho-convex sets in the plane

Topological properties of connected ortho-convex sets in the plane, i.e., connected sets convex along the horizontal and vertical lines are studied. Several geometric statements concerning the ortho-separation of ortho-convex sets are proved.     

Relatively convex subsets of simply connected planar sets

Let D ⊂ ℜ² be simply connected. A subset K ⊂ D is relatively convex if a, b ∈ K, [a, b] ⊂ D implies [a, b] ⊂ K. We establish the following version of Helly’s Topological Theorem: If is a family of (at least 3) compact, polygonally connected and relatively convex subsets of D, then , provided each three members of meet. We also prove other results related to the combinatorial metric ρ_K(a, b) (= smallest number of edges of a polygonal path from a to b in K).     

On trees and tree dimension of ordered sets

We call an ordered set (X, ) a tree if no pair of incomparable elements ofX has an upper bound. It is shown that there is a natural way to associate a tree (T, ) with any ordered set (X, ), and (T, ) can be characterized by a universal property. We define the tree dimensiontd(X, ) of an ordered set as the minimal number of extensions of (X, ) which are trees such that the given order is the intersection of those tree orders. We give characterizations of the tree dimension, relations between dimension and tree dimension, and removal theorems.     

Context-free pairs of groups II - Cuts, tree sets, and random walks

This is a continuation of the study, begun by Ceccherini-Silberstein and Woess (2009) [5], of context-free pairs of groups and the related context-free graphs in the sense of Muller and Schupp (1985) [22]. The graphs under consideration are Schreier graphs of a subgroup of some finitely generated group, and context-freeness relates to a tree-like structure of those graphs. Instead of the cones of Muller and Schupp (1985) [22] (connected components resulting from deletion of finite balls with respect to the graph metric), a more general approach to context-free graphs is proposed via tree sets consisting of cuts of the graph, and associated structure trees. The existence of tree sets with certain "good" properties is studied. With a tree set, a natural context-free grammar is associated. These investigations of the structure of context free pairs, resp. graphs are then applied to study random walk asymptotics via complex analysis. In particular, a complete proof of the local limit theorem for return probabilities on any virtually free group is given, as well as on Schreier graphs of a finitely generated subgoup of a free group. This extends, respectively completes, the significant work of Lalley (1993, 2001) [18,20].     

The number of maximal independent sets in connected graphs

Generalizing a theorem of Moon and Moser, we determine the maximum number of maximal independent sets in a connected graph on n vertices for n sufficiently large, e.g., n > 50.     

Arcwise connectedness of the boundary of connected self-similar sets

Summary Let Tbe the connected attractor of injective contractions f₁,... f_mon R^d that satisfy the Open Set Condition. We show that ∂Tis arcwise connected. In particular, the boundary of the Levy dragon and those of the fundamental domains of canonical number systems are arcwise connected.