Publications

We show that if κ is a Ramsey cardinal then every fine countably complete normal ideal concentrating on a set in V_κ (hence every normal ideal on ω₁) satisfies a weak form of precipitousness. As an application we show that Ramsey cardinals can be used instead of Woodin cardinals and stationary tower forcing to push through the Raghavan-Todorčević proof of a longstanding conjecture of Galvin: if a Ramsey cardinal exists then for any uncountable set of reals X and any coloring c of [X]² with finitely many colors, there is a subspace of X homeomorphic to the rationals on which c takes on at most two colors. Finally, we use work of Donder and Levinsky to show that this partition theorem for uncountable sets of reals can hold in a generic extension of Gödel's L, and therefore does not imply the existence of 0^♯.

The purpose of this article is to give new constructions of linear orders which are minimal with respect to being non-σ-scattered. Specifically, we will show that Jensen's principle ♢ implies that there is a minimal Countryman line, answering a question of Baumgartner. We also produce the first consistent examples of minimal non-σ-scattered linear orders of cardinality greater than ℵ₁, as given a successor cardinal κ+, we obtain such linear orderings of cardinality κ+ with the additional property that their square is the union of κ-many chains. We give two constructions: directly building such examples using forcing, and also deriving their existence from combinatorial principles. The latter approach shows that such minimal non-σ-scattered linear orders of cardinality κ+ exist for every cardinal κ in Gödel's constructible universe, and also (using work of Rinot) that examples must exist at successors of singular strong limit cardinals in the absence of inner models satisfying the existence of a measurable cardinal μ of Mitchell order μ++.