Basis problems for uncountable linear orders and trees
J. Moore proved the consistency of five element basis. In this talk, we will prove the consistency of 3+2n element basis for n>1.
We will use the following concept introduced by S. Todorcevic. For a special coherent tree T, I(T) is the ideal consists of subsets of ω1 such that on which, some uncountable subset never splits.
For some collection ⇀Γ of pairwise I(T)-disjoint I(T)-positive sets. PFA(⇀Γ) is the assertion that for any proper forcing that does not destroy I(T)-positivity of any set in ⇀Γ, for any ω1 collection of dense sets, there is a filter meets them all.
We will first prove the consistency of PFA(⇀Γ) from a supercompact cardinal. Then prove that 3+2n element basis follows from PFA(⇀Γ) where n is the size of ⇀Γ.
If we have only one Aronszajn tree in the final model, then 2n is the best we can do for Countryman basis. To get more posibilities, we may want to construct a model with more than one (probably finite) trees. Basis for Aronszajn trees will also be discussed.
This is a joint work with L. Wu.