Blog Archives

Topic Archive: generic absoluteness

Set theory seminarFriday, October 23, 201510:00 amGC 3212

Scott Cramer

Tree representations from very large cardinals

Rutgers University

We will discuss the propagation of certain tree representations in the presence of very large cardinals. These tree representations give generic absoluteness results and have structural consequences in the area of generalized descriptive set theory. In fact these representations give us a method for producing models of strong determinacy axioms.

Set theory seminarFriday, December 6, 201310:00 am

Thomas Johnstone

Bagaria’s characterization of bounded forcing axioms in terms of generic absoluteness

The New York City College of Technology (CityTech), CUNY

Goldstern and Shelah (1995) introduced the class of bounded forcing axioms, that is forcing axioms for families of antichains of bounded size. For example, the bounded proper forcing axiom ${\rm BPFA}$ asserts that for any proper forcing notion $\mathbb{P}$ and any collection $D$ of at most $\aleph_1$ many maximal antichains in $\mathbb{P}$, each of size at most $\aleph_1$, there is a filter on $\mathbb{P}$ meeting each antichain in $D$. The speaker will present a theorem of Joan Bagaria (2000) that characterizes bounded forcing axioms in terms of generic absoluteness: for instance, Bagaria’s result shows that ${\rm BPFA}$ is equivalent to the assertion that if a $\Sigma_1$ sentence of the language of set theory with parameters of hereditary size at most $\aleph_1$ is true in some proper forcing extension, then it is already true in the ground model.