# Blog Archives

# Topic Archive: maximality principle

# Subcomplete forcings

Subcomplete forcings are a class of forcings introduced by Jensen. These forcings do not add reals but may change cofinalities to $\omega$, unlike proper forcings. Examples of subcomplete forcings include Namba forcing, Prikry forcing, and any countably closed forcing. In this talk I will discuss some results concerning subcomplete forcing and the preservation of various properties of trees.

# Generalized Baire spaces and closed Maximality Principles

Given an uncountable regular cardinal $\kappa$, the generalized Baire space of $\kappa$ is set ${}^\kappa\kappa$ of all functions from $\kappa$ to $\kappa$ equipped with the topology whose basic open sets consist of all extensions of partial functions of cardinality less than $\kappa$.

A subset of this space is $\mathbf{\Sigma}^1_1$ (i.e. a projection of a closed subset of ${}^\kappa\kappa\times{}^\kappa\kappa$) if and only it is definable over $\mathrm{H}(\kappa^+)$ by a $\Sigma_1$-formula with parameters. This shows that the class of $\mathbf{\Sigma}^1_1$-subsets contains a great variety of set-theoretically interesting objects. Moreover, it is known that many basic and interesting questions about sets in this class are not decided by the axioms of $\mathrm{ZFC}$ plus large cardinal axioms.

In my talk, I want to present examples of extensions of $\mathrm{ZFC}$ that settle many of these questions by providing a nice structure theory for the class of $\mathbf{\Sigma}^1_1$-subsets of ${}^\kappa\kappa$. These forcing axioms appear in the work of Fuchs, Leibman, Stavi and Väänänen. They are variations of the *maximality principle* introduced by Stavi and Väänänen and later rediscovered by Hamkins.