The proposed research shall advance our knowledge in the field of set theory, a subfield of mathematical logic. Mildenberger proposes to work together with Shelah on some combinatorial questions in an area where independence of the axioms of mathematics, i.e., the Zermelo Fraenkel axiom system with together with the axiom of choice, short ZFC, is very likely. Therefore the main part of the proposed work is to develop forcing techniques. Combinatorial methods in the analysis of existing notions of forcing with respect to new properties would also be emphasised. In the proposed work, cardinal characteristics of the continuum often encapsulate important combinatorial features of the ZFC models in question. A cardinal characteristic of the continuum locates the smallest size of a set with a property that is usually not exhibited by any countable set and that is exhibited by at least one set of size of the continuum. However, sometimes a mathematical statement is derived from some delicate stratification of the set-theoretic universe that cannot (yet) be reduced to cardinal equations or inequalities. This can in particular be the case for set-theoretic universes that we intend to construct with not so conventional forcing constructions, such as non-linear forcing iterations and iterations with partial memory. We also propose to investigate forcings with oracle chain conditions. We want to advance these techniques further and develop new forcing constructions that will be useful for some longstanding open questions. The proposed research has applications to open problems in topology, algebra and in the combinatorics of the powerset of aleph_1. We propose to investigate the possible number of near-coherence classes of ultrafilters, some combinatorial properties of semifilters, and the connection between guessing principles and the existence of Souslin trees.
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