Sequential and distributive forcings without choice

Abstract

AbstractIn the Zermelo–Fraenkel set theory with the Axiom of Choice, a forcing notion is “ $\kappa $ -distributive” if and only if it is “ $\kappa $ -sequential.” We show that without the Axiom of Choice, this equivalence fails, even if we include a weak form of the Axiom of Choice, the Principle of Dependent Choice for $\kappa $ . Still, the equivalence may still hold along with very strong failures of the Axiom of Choice, assuming the consistency of large cardinal axioms. We also prove that although a $\kappa $ -distributive forcing notion may violate Dependent Choice, it must preserve the Axiom of Choice for families of size $\kappa $ . On the other hand, a $\kappa $ -sequential can violate the Axiom of Choice for countable families. We also provide a condition of “quasiproperness” which is sufficient for the preservation of Dependent Choice, and is also necessary if the forcing notion is sequential.