On sets with Baire property in topological spaces

Steinhaus [9] prove that if a set A has a positive Lebesgue measure in the line then its distance set contains an interval. He obtained even stronger forms of this result in [9], which are concerned with mutual distances between points in an infinite sequence of sets. Similar theorems in the case we replace distance by mutual ratio were established by Bose-Majumdar [1]. In the present paper, we endeavour to obtain some results related to sets with Baire property in locally compact topological spaces, particular cases of which yield the Baire category analogues of the above results of Steinhaus [9] and their corresponding form for ratios by Bose-Majumdar [1].     

On the difference property of the family of functions with the Baire property

We show that it is consistent with ZFC that the family of functions with the Baire property has the difference property. That is, every function for which f(x + h)-f(x) has the Baire property for every h∈R is of the form f=g + Awhere g has the Baire property and A is additive. This revised version was published online in June 2006 with corrections to the Cover Date.     

On indexed families of multifunctions generated by families of functions

We present some necessary and some sufficient conditions for a family of multifunctions generated by two families of real functions to be a collapsing, an expanding family or an iteration semigroup. Some properties of set-valued iteration groups generated by commuting homeomorphisms not embeddable in an iteration group are given.     

Measurability and the abstract Baire property

Within an abstract theory of point sets the author has successfully unified a substantial number of the analogous theorems concerning Lebesgue measure and Baire category. It has been shown that the Lebesgue measurable sets coincide with the sets having the abstract Baire property with respect to the family of all closed sets of positive Lebesgue measure and the question was raised in [2] whether the sets measurable with respect to certain Hausdorff measures were the same as the sets having the abstract Baire property with respect to the family of all closed sets of positive Hausdorff measure. In this article we establish a general theorem which, under the assumption of the continuum hypothesis, gives an affirmative answer to this question.     

On ramsey families of sets

The main result of this paper is a lemma which can be used to prove the existence of highchromatic subhypergraphs of large girth in various hypergraphs. In the last part of the paper we use amalgamation techniques to prove the existence for everyl, k 3 of a setA of integers such that the hypergraph having as edges all the arithmetic progressions of lengthk inA has both chromatic number and girth greater thanl.     

On families of intersecting sets

While algorithms exist which produce optimal binary trees, there are no direct formulas for the cost of such trees. In this note, we give a formula for the cost of the optimal binary tree built on m nodes with weights 1, 2, 3,…, m. The simplicity of this proof suggests that one can try to compute the cost of optimal trees for other special classes of weights.     

Some properties of Baire sets and sets of positive measure

In un lavoro classico Steinhaus ha dimostrato undici teoremi interessanti nei quali si considerano insiemi distanziali di sottoinsiemi della linea reale,aventi misura di Lebesgue positiva. Majumder ha dimostrato che sono validi teoremi analoghi a quelli di Steinhaus per insiemi del tipoR(E)={x/y: x,y∈E} doveE∪R; 0?E. In questo lavoro si considerano funzioni generali e si investiga sui quali degli undici teoremi di Steinhaus possono essere generalizzati. Nell'articolo, inoltre, usando una funzione generalef, vengono dati risultati analoghi a quelli di Steinhaus per insiemi di Baire.     

Baire property and axiom of choice

We show that without using inaccessible cardinals it is possible to get models of “ZF+all sets of reals have the Baire property +DC(ω₁)” and “ZFC+all projective sets have the Baire property+the union of less than ω₂ many meager sets is meager”, answering two well-known open questions of Woodin and Judah, respectively. The authors would like to thank the Israel Academy of Sciences BSF for partial support. The second author would like to thank the Landau Center for Mathematical Analysis, supported by the Minerva Foundation (Germany).     

On intersecting families of finite sets

Let X = [1, n] be a finite set of cardinality n and let $\text{F}$ be a family of k-subsets of X. Suppose that any two members of $\text{F}$ intersect in at least t elements and for some given positive constant c, every element of X is contained in less than c |$\text{F}$| members of $\text{F}$. How large |$\text{F}$| can be and which are the extremal families were problems posed by Erdös, Rothschild, and Szemerédi. In this paper we answer some of these questions for n > n₀(k, c). One of the results is the following: let $\text{t = 1,}\text{3}\text{7}\text{< c <}\text{1}\text{2}$. Then whenever $\text{F}$ is an extremal family we can find a 7-3 Steiner system $\text{B}$ such that $\text{F}$ consists exactly of those k-subsets of X which contain some member of $\text{B}$.     

Covering the Baire space by families which are not finitely dominating

It is consistent (relative to ZFC) that each union of many families in the Baire space which are not finitely dominating is not dominating. In particular, it is consistent that for each nonprincipal ultrafilter , the cofinality of the reduced ultrapower is greater than . The model is constructed by oracle chain condition forcing, to which we give a self-contained introduction.     

On a property of polyhedral sets

The following result is proved using solvability and optimality criteria for linear programs. The duals to the cones of feasible directions at vertices of a polyhedral set constitute a partition of the dual to the recession cone of the set.