Openly generated Boolean algebras and the Fodor-type reflection principle

Joint work with Sakaé Fuchino.

Abstract: We prove that the Fodor-type Reflection Principle (FRP) is equivalent to the assertion that any Boolean algebra is openly generated if and only if it is $\aleph _2$-projective. Previously it was known that this characterization of openly generated Boolean algebras follows from Axiom R.

Since FRP is preserved by c.c.c. generic extension, we conclude in particular that this characterization is consistent with any set-theoretic assertion forcable by a c.c.c. poset starting from a model of FRP.

A crucial step of the proof of the main result is to show that FRP implies Shelah’s Strong Hypothesis (SSH). In particular, we show that FRP implies the Singular Cardinals Hypothesis (SCH). Extending a result of Rinot, we also establish some new characterizations of SSH in terms of topological reflection theorems.

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