Ketonen’s question and other cardinal sins

Joint work with Zhixing You and Jiachen Yuan.

Abstract. Intersection models of generic extensions obtained from a commutative projection systems of notions of forcing has recently regained interest, especially in the study of descriptive set theory. Here, we show that it provides a fruitful framework that opens the door to solving some open problems concerning compactness principles of small cardinals. To exemplify, from suitable assumptions, we construct intersection models satisfying ZFC and any of the following:

• There is a weakly compact cardinal $\kappa$ carrying an indecomposable ultrafilter, yet $\kappa$ is not measurable. This answers a question of Ketonen from the late 1970’s.
• For proper class many cardinals $\lambda$, the least $\lambda$-strongly compact cardinal is singular. This answers a question of Bagaria and Magidor who asked for merely two such cardinals.
• There is a strongly inaccessible cardinal whose $C$-sequence number is a singular cardinal. This answers a question of Lambie-Hanson and the first author.

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