Forcing, games and families of closed sets

Forcing properties of ideals of closed sets

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...

Union-closed families of sets

A family of sets is union-closed if it contains the union of any two of its elements. Some years ago, Reimer gave a lower bound for the average size of an element of a union-closed family consisting of m sets and, two years ago, Czédli did the same under the additional condition that our sets are contained in a set with n elements. Recently Tom Eccles and I have determined the minimum average s...

Forcing with Ideals Generated by Closed Sets

Consider the posets PI = Borel(R) \ I where I is a σ-ideal σ-generated by a projective collection of closed sets. Then the PI extension is given by a single real r of an almost minimal degree: every real s ∈ V [r] is Cohen-generic over V or

Learning Families of Closed Sets in Matroids

On the Characterization of some Families of Closed Convex Sets∗

This paper deals with the characterization of the sums of compact convex sets with linear subspaces, simplices, sandwiches (convex hulls of pairs of parallel affine manifolds) and parallelotopes in terms of the so-called internal and conical representations, topological and geometrical properties. In particular, it is shown that a closed convex set is a sandwich if and only if its relative boun...