Joel Hamkins: What is the theory ZFC-Powerset?

Toronto Set Theory Seminar
Friday, April 22 from 1:30 to 3pm
Fields Institute, Room 210

Speaker: Joel Hamkins (CUNY)

Title: What is the theory ZFC-Powerset?

Abstract: The theory ZFC-, consisting of the usual axioms of ZFC but with the Powerset axiom removed, when axiomatized by Extensionality, Foundation, Pairing, Union, Infinity, Separation, Replacement and the Axiom of Choice, is weaker than commonly supposed, and suffices to prove neither that a countable union of countable sets is countable, nor that $omega_1$ is regular, nor that the Los theorem holds for ultrapowers, even for well-founded ultrapowers on a measurable cardinal, nor that the Gaifman theorem holds, that is, that every $Sigma_1$-elementary cofinal embedding $j:Mto N$ between models of the theory is fully elementary, nor that $Sigma_n$ sets are closed under bounded quantification. Nevertheless, these deficits of ZFC- are completely repaired by strengthening it to the theory obtained by using Collection rather than Replacement in the axiomatization above. These results extend prior work of Zarach. This is joint work with Victoria Gitman and Thomas Johnstone.

One response to “Joel Hamkins: What is the theory ZFC-Powerset?

  1. Samuel Coskey

    The Institute will be closed on this date. Please come to the back door of the Institute before the seminar begins to be let in. You must identify yourself to be let in.

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