A Mitchell-like order for Ramsey and Ramsey-like cardinals

TL;DR

This paper introduces a Mitchell-like order for Ramsey and Ramsey-like cardinals, establishing a new hierarchy with desirable properties and analyzing its behavior under forcing extensions.

Contribution

It defines a novel Mitchell-like order for small large cardinals and demonstrates its properties and robustness under forcing, extending the understanding of Ramsey-like cardinals.

Findings

The Mitchell-like order has properties similar to the classical Mitchell order.

Forcing extensions with cover and approximation properties do not increase the rank.

Techniques for soft killing of large-cardinal degrees are applied to Ramsey-like cardinals.

Abstract

Smallish large cardinals $\kappa$ are often characterized by the existence of a collection of filters on $\kappa$, each of which is an ultrafilter on the subsets of $\kappa$ of some transitive $\mathrm{ZFC}^-$-model of size $ \kappa$. We introduce a Mitchell-like order for Ramsey and Ramsey-like cardinals, ordering such collections of small filters. We show that the Mitchell-like order and the resulting notion of rank have all the desirable properties of the Mitchell order on normal measures on a measurable cardinal. The Mitchell-like order behaves robustly with respect to forcing constructions. We show that extensions with cover and approximation properties cannot increase the rank of a Ramsey or Ramsey-like cardinal. We use the results about extensions with cover and approximation properties together with recently developed techniques about soft killing of large-cardinal degrees by forcing to softly kill the ranks of Ramsey and Ramsey-like cardinals.