An Ordered Set of Size ℵ1 with Monochromatic Maximal Chains

Every model of ZFC contains a product of two linear orders, each of size ₁, with the property that every subset or complement thereof contains a maximal chain.     

Strengthened fixed point property and products in ordered sets

Strengthened fixed point property for ordered sets is formulated. It is weaker than the strong fixed point property due to Duffus and Sauer and stronger than the product property meaning that A × Y has the fixed point property whenever A has the former and Y has the latter. In particular, doubly chain complete ordered sets with no infinite antichain have the strengthened fixed point property whenever they have the fixed point property, which yields a transparent proof of the well-known theorem saying that doubly chain complete ordered sets with no infinite antichain have the product property whenever they have the fixed point property. The new proof does not require the axiom of choice. Presented at the Summer School on General Algebra and Ordered Sets, Malá Morávka, 4–10 September 2005.     

On the size of maximal antichains and the number of pairwise disjoint maximal chains

Fix integers n and k with n≥k≥3. Duffus and Sands proved that if P is a finite poset and n≤|C|≤n+(n−k)/(k−2) for every maximal chain in P, then P must contain k pairwise disjoint maximal antichains. They also constructed a family of examples to show that these inequalities are tight. These examples are two-dimensional which suggests that the dual statement may also hold. In this paper, we show that this is correct. Specifically, we show that if P is a finite poset and n≤|A|≤n+(n−k)/(k−2) for every maximal antichain in P, then P has k pairwise disjoint maximal chains. Our argument actually proves a somewhat stronger result, and we are able to show that an analogous result holds for antichains.     

Combinatorial Criteria for Ramifiable Ordered Sets

The tree‐property (classica in cardina theory) and its variants make sense also for directed sets and even for partially ordered sets. A combinatoria approach is developed here, with characterizations and criteria involving (inter alia) adequate families of special substructures of directed sets. These substructures form a natural hierarchy that is also investigated.     

On shadowing property for inverse limit spaces

We study here the G-shadowing property of the shift map σ on the inverse limit space X _f, generated by an equivariant self-map f on a metric G-space X.      

Coupled fixed point results for mappings without mixed monotone property in partially ordered G-metric spaces

In this paper, we prove coupled fixed point results for mappings without mixed monotone property in partially ordered G-metric spaces. Also we showed that if $(X\text{,}G)$ is regular, these fixed point results holds.     

On poset similarity

Let P be a poset, and let A be an element of its strict incidence algebra. Saks (SIAM J. Algebraic Discrete Methods 1 (1980) 211–215; Discrete Math. 59 (1986) 135–166) and Gansner (SIAM J. Algebraic Discrete Methods 2 (1981) 429–440) proved that the kth Dilworth number of P is less than or equal to the dimension of the nullspace of A^k, and that there is some member of the strict incidence algebra of P for which equality is attained (for all k simultaneously). In this paper we focus attention on the question of when equality is attained with the strict zeta matrix, and proceed under a particular random poset model. We provide an invariant depending only on two measures of nonunimodality of the level structure for the poset that, with probability tending to 1 as the smallest level tends to infinity, takes on the same value as the inequality gap between the width of P and the dimension of the nullspace of its strict zeta matrix. In particular, we characterize the level structures for which the width of P is, with probability tending to 1, equal to the dimension of the nullspace of its strict zeta matrix. As a consequence, by the Kleitman–Rothschild Theorem 5, almost all posets in the Uniform random poset model have width equal to the dimension of the nullspace of their zeta matrices. We hope this is a first step toward a complete characterization of when equality holds in Saks’ and Gansner's inequality for the strict zeta matrix and for all k. New to this paper are also the canonical representatives of the poset similarity classes (where two posets are said to be similar if their strict zeta matrices are similar in the matrix-theoretic sense), and these form the setting for our work on Saks’ and Gansner's inequalities. (Also new are two functions that measure the nonunimodality of a sequence of real numbers.)     

On Finite Groups with a Given Number of Centralizers

For a finite group G, let Cent(G) denote the set of centralizers of single elements of G and #Cent(G) = |Cent(G)|. G is called an n-centralizer group if #Cent(G) = n, and a primitive n-centralizer group if #Cent(G) = #Cent(G/Z(G)) = n. In this paper, we compute #Cent(G) for some finite groups G and prove that, for any positive integer n 2, 3, there exists a finite group G with #Cent(G) = n, which is a question raised by Belcastro and Sherman [2]. We investigate the structure of finite groups G with #Cent(G) = 6 and prove that, if G is a primitive 6-centralizer group, then G/Z(G) A₄, the alternating group on four letters. Also, we prove that, if G/Z(G) A₄, then #Cent(G) = 6 or 8, and construct a group G with G/Z(G) A₄ and #Cent(G) = 8.This research was in part supported by a grant from IPM.2000 Mathematics Subject Classification: 20D99, 20E07     

Order-couniversality of the complete infinitary tree: An application of zero-divisor graphs

Let

be the class of countably infinite bounded partially ordered sets

such that every non-minimum element of

has only finitely many successors, and has infinitely many immediate predecessors. Write

for the poset obtained by introducing maximum and minimum elements to the complete infinitary tree of nonempty finite sequences

of positive integers, where

if

is an extension of

. A poset

is called

-couniversal if

and for every

there is a bijective poset-homomorphism

. In this paper, couniversality is linked to zero-divisor graphs of partially ordered sets. It is proved that

is

-couniversal if and only if every non-maximum element of

is a (poset-theoretic) zero-divisor of

, and the zero-divisor graph of

is a spanning subgraph of the zero-divisor graph of

.     

The finite cutset property

A cutset of H is a subset of ∪ H which meets every element of H. H has the finite cutset property if every cutset of H contains a finite one. We study this notion, and in particular how it is related to the compactness of H for the natural topology. MSC: 04A20, 54D30.     

On Rusakov's n-ary rs-Groups

Properties of n-ary groups connected with the affine geometry are considered. Some conditions for an n-ary rs-group to be derived from a binary group are given. Necessary and sufficient conditions for an n-ary group -derived from an additive group of a field to be an rs-group are obtained. The existence of non-commutative n-ary rs-groups which are not derived from any group of arity m<n for every n3, r>2 is proved.     

On Pseudocomplemented and Stone Ordered Sets

The aim of this paper is to characterize both the pseudocomplemented and Stone ordered sets in a manner similar to that used previously for Boolean and distributive ordered sets. The sublattice G(A) of the Dedekind–Mac Neille completion DM(A) of an ordered set A generated by A is said to be the characteristic lattice of A. We will show that there are distributive pseudocomplemented ordered sets whose characteristic lattices are not pseudocomplemented. We can define a stronger notion of pseudocomplementedness by demanding that both A and G(A) be pseudocomplemented. It turns out that the two concepts are the same for finite and Stone ordered sets.     

On the best linear approximation methods and the widths of certain classes of analytic functions

We discuss the best linear approximation methods in the Hardy spaceH _q q≥1, for classes of analytic functions studied by N. Ainulloev; these are generalizations (in a certain sense) of function sets introduced by L. V. Taikov. The exact values of their linear and Gelfandn-widths are obtained. The exact values of the Kolmogorov and Bernsteinn-widths of classes of analytic (in |z|<1) functions whose boundaryK-functionals are majorized by a prescribed functions are also obtained. Translated fromMatermaticheskie Zametki, Vol. 65, No. 2, pp. 186–193, February, 1999.     

偏序集的关联环的同构问题

设Ｘ是局部有限偏序集（或拟序集），Ｒ是含１的结合环，Ⅰ（Ｘ，Ｒ）是Ｒ上Ｘ的关联环，关联环的同构问题是指：问题１：怎样的环，能使环同构Ⅰ（Ｘ，Ｒ）Ⅰ（Ｘ，Ｒ）推出偏序集之间的同构Ｘ芒Ｘ’？问题２：怎样的环或偏序集，能使环同构Ⅰ（Ｘ，Ｒ）Ⅰ（Ｘ，Ｓ）推出Ｒ　Ｓ？本文证明了对唯一幂等元环（非交换），问题１有正面回答；对问题２，我们证明了对交换不可分解环Ｒ、Ｓ，由环同构Ⅰ（Ｘ，Ｒ）Ⅰ（Ｘ，Ｒ）可得到Ｒ＝Ｓ，Ｘ＝Ｘ’。     

Can transitive orientation make sandwich problems easier?

A graph G_s=(V,E_s) is a sandwich for a pair of graphs G_t=(V,E_t) and G=(V,E) if E_t⊆E_s⊆E. A sandwich problem asks for the existence of a sandwich graph having an expected property. In a seminal paper, Golumbic et al. [Graph sandwich problems, J. Algorithms 19 (1995) 449-473] present many results on sub-families of perfect graphs. We are especially interested in comparability (resp., co-comparability) graphs because these graphs (resp., their complements) admit one or more transitive orientations (each orientation is a partially ordered set or poset). Thus, fixing the orientations of the edges of G_t and G restricts the number of possible sandwiches. We study whether adding an orientation can decrease the complexity of the problem. Two different types of problems should be considered depending on the transitivity of the orientation: the poset sandwich problems and the directed sandwich problems. The orientations added to both graphs G and G_s are transitive in the first type of problem but arbitrary for the second type.     

Irreducible Width 2 Posets of Linear Discrepancy 3

The linear discrepancy of a poset P is the least k such that there is a linear extension L of P such that if x and y are incomparable in P, then |h _L (x) − h _L (y)| ≤ k, where h _L (x) is the height of x in L. Tannenbaum, Trenk, and Fishburn characterized the posets of linear discrepancy 1 as the semiorders of width 2 and posed the problem for characterizing the posets of linear discrepancy 2. Howard et al. (Order 24:139–153, 2007) showed that this problem is equivalent to finding all posets of linear discrepancy 3 such that the removal of any point reduces the linear discrepancy. In this paper we determine all of these minimal posets of linear discrepancy 3 that have width 2. We do so by showing that, when removing a specific maximal point in a minimal linear discrepancy 3 poset, there is a unique linear extension that witnesses linear discrepancy 2. The first author was supported during this research by National Science foundation VIGRE grant DMS-0135290.     

Shuffling Chromosomes

The gene order of chromosomes can be rearranged by chromosomal inversions that reverse the order of segments. Motivated by a comparative study of two Drosophila species, we investigate the number of reversals that are needed to scramble the gene order when all reversals are equally likely and when the segments reversed are never more than L genes. In studying this question we prove some new results about the convergence to equilibrium of shuffling by transposition and the one dimensional simple exclusion process.     

On a certain property of the BurkillSCP-integral

We establish a necessary condition for theSCP-integrability of a function and use it to construct an example of a function possessing a continuousSCP-majorant andSCP-minorant on a closed interval, but nonintegrable over this interval in the sense of theSCP-integral. Translated fromMatematicheskie Zametki, Vol. 65, No. 4, pp. 599–606, April, 1999.