To each closed subset S of a finite dimensional Euclidean space corresponds a σ-ideal of sets J (S) which is σ-generated over S by the convex subsets of S. The set-theoretic properties of this ideal hold geometric information about the set. We discuss the relation of reducibility between convexity ideals and the connections between convexity ideals to other types of ideals, such as the ideals w...

A function f from a countable product X = ∏ i Xi of Polish spaces into a Polish space is separately nowhere constant provided it is nowhere constant on every section of X. We show that every continuous separately nowhere constant function is one-to-one on a product of perfect subsets of Xi’s. This result is used to distinguish between n-cube density notions for different n ≤ ω, where ω-cube den...

We show that every locally finite bipartite Borel graph satisfying a strengthening of Hall’s condition has a Borel perfect matching on some comeager invariant Borel set. We apply this to show that if a group acting by Borel automorphisms on a Polish space has a paradoxical decomposition, then it admits a paradoxical decomposition using pieces having the Baire property. This strengthens a theore...

Given a graph G whose set of vertices is a Polish space X, the weak Borel chromatic number of G is the least size of a family of pairwise disjoint G-independent Borel sets that covers all of X. Here a set of vertices of a graph G is independent if no two vertices in the set are connected by an edge. We show that it is consistent with an arbitrarily large size of the continuum that every closed ...

Here θ is a probability measure on a Polish space , Dr k k = 1 2r is a dyadic partition of (hence the use of 2r summands) satisfying θ Dr k = 1/2r and Lq 1 Lq 2 Lq 2r is an independent, identically distributed sequence of random probability measures on a Polish space such that Lq k q ∈ N satisfies the large deviation principle with a convex rate function. A number of related asymptotic results ...

This article is devoted to the study of continuous colorings of the n-element subsets of a Polish space. The homogeneity number hm(c) of an n-coloring c : [X]n → 2 is the least size of a family of c-homogeneous sets that covers X. An n-coloring is uncountably homogeneous if hm(c) > א0. Answering a question of B. Miller, we show that for every n > 1 there is a finite family B of continuous n-col...

With every σ-ideal I on a Polish space we associate the σ-ideal generated by closed sets in I. We study the forcing notions of Borel sets modulo the respective σ-ideals and find connections between their forcing properties. To this end, we associate to a σ-ideal on a Polish space an ideal on a countable set and show how forcing properties of the forcing depend on combinatorial properties of the...

A weakly continuous near-action of a Polish group G on a standard Lebesgue measure space (X,μ) is whirly if for every A ⊆ X of strictly positive measure and every neighbourhood V of identity in G the set V A has full measure. This is a strong version of ergodicity, and locally compact groups never admit whirly actions. On the contrary, every ergodic near-action by a Polish Lévy group in the sen...