Norms on possibilities I: forcing with trees and creatures

by Roslanowski and Shelah. [RoSh:470]
Memoirs American Math Soc, 1999
In this paper we present a systematic study of the method of norms on possibilities of building forcing notions with keeping their properties under full control. This technique allows us to answer several open problems, but on our way to get the solutions we develop various ideas interesting per se. These include a new iterable condition for ``not adding Cohen reals'' (which has a flavour of preserving special properties of p-points), new intriguing properties of ultrafilters (weaker than being Ramsey but stronger than p --point) and some new applications of variants of the PP--property.

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