# Blog Archives

# Topic Archive: Ramsey cardinals

# The Mitchell order for Ramsey cardinals

The usual Mitchell relation on normal measures on a measurable cardinal $\kappa$ orders the measures based on the degree of measurability that $\kappa$ retains in their respective ultrapowers. We shall examine the analogous ordering of appropriate witnessing objects for Ramsey (and Ramsey-like) cardinals. It turns out that the resulting order is well-behaved and its degrees neatly stratify the large cardinal hierarchy between Ramsey, strongly Ramsey, and super Ramsey cardinals. We also give a soft killing argument for this notion of Mitchell rank.

This is joint work with Victoria Gitman and Erin Carmody.

# Ramsey cardinals and the continuum function

In his famous theorem, Easton used the Easton Product forcing to show that if $V\models{\rm GCH}$ and $F$ is any weakly increasing function on the regular cardinals such that $\text{cf}(F(\alpha))>\alpha$, then there is a cofinality preserving forcing extension in which $F$ is realized as the continuum function. The investigation then shifted to identifying which continuum patterns are compatible with large cardinals. It is not difficult to see that large cardinals affect the behavior of the continuum function. Obviously, if $\kappa$ is inaccessible, then by definition, the continuum function must have a closure at $\kappa$. Some other large cardinal influences are much more subtle. Easton’s original forcing does not work well in the presence of large cardinals; it, for instance, destroy weak compactness over $L$. So set theorists have had to develop some general and other very specific forcing techniques to address the behavior of the continuum function for a given large cardinal. In this talk, we will show that if $V\models{\rm GCH}$, $\kappa$ is Ramsey, and $F$ is any weakly increasing class function on the regular cardinals with a closure point at $\kappa$ such that $\text{cf}(F(\alpha))>\alpha$, then there is a cofinality preserving forcing extension in which $\kappa$ remains Ramsey and $F$ is realized as the continuum function. This is joint work with Brent Cody.

An extended abstract can be found here.

# Indestructibility for Ramsey cardinals

A large cardinal $\kappa$ is said to be indestructible by a certain poset $\mathbb P$ if $\kappa$ retains the large cardinal property in all forcing extensions by $\mathbb P$. Since most relative consistency results for ${\rm ZFC}$ are obtained via forcing, the knowledge of a large cardinal’s indestructibility properties is used to establish the consistency of that large cardinal with other set theoretic properties. In this talk, I will use an elementary embeddings characterization of Ramsey cardinals to prove some basic indestructibility results.

# An elementary embeddings characterization of Ramsey cardinals

A cardinal $\kappa$ is Ramsey if every coloring of the finite subsets of $\kappa$ in two colors has a homogeneous set of size $\kappa$. In this talk, I will motivate and prove some basics facts about the little known but very elegant and useful elementary embeddings characterization of Ramsey cardinals.