Specializing trees and answer to a question of Williams

Mohammad Golshani; Saharon Shelah

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arXiv · 1708.02719

Specializing trees and answer to a question of Williams

Mohammad Golshani·Saharon Shelah

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Abstract

We show that if $cf(2^{\aleph_0})=\aleph_1,$ then any non-trivial $\aleph_1$-closed forcing notion of size $\leq 2^{\aleph_0}$ is forcing equivalent to $Add(\aleph_1, 1),$ the Cohen forcing for adding a new Cohen subset of $\omega_1.$ We also produce, relative to the existence of suitable large cardinals, a model of $ZFC$ in which $2^{\aleph_0}=\aleph_2$ and all $\aleph_1$-closed forcing notion of size $\leq 2^{\aleph_0}$ collapse $\aleph_2,$ and hence are forcing equivalent to $Add(\aleph_1, 1).$ These results answer a question of Scott Williams from 1978. We also extend a result of Todorcevic and Foreman-Magidor-Shelah by showing that it is consistent that every partial order which adds a new subset of $\aleph_2,$ collapses $\aleph_2$ or $\aleph_3.$

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Mohammad Golshani, Saharon Shelah. 2017-08-09. Specializing trees and answer to a question of Williams. https://arxiv.org/abs/1708.02719

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