# Ramsey cardinal

Jump to navigation Jump to search

In mathematics, a Ramsey cardinal is a certain kind of large cardinal number introduced by Erdős & Hajnal (1962) and named after Frank P. Ramsey.

Let [κ] denote the set of all finite subsets of κ. Then a cardinal number κ is called Ramsey if, for every function

f: [κ] → {0, 1}

there is a set A of cardinality κ that is homogeneous for f. That is, for every n, f is constant on the subsets of cardinality n from A. A cardinal κ is called ineffably Ramsey if A can be chosen to be stationary subset of κ. A cardinal κ is called virtually Ramsey if for every function

f: [κ] → {0, 1}

there is C, a closed and unbounded subset of κ, so that for every λ in C of uncountable cofinality, there is an unbounded subset of λ, which is homogenous for f; slightly weaker is the notion of almost Ramsey where homogenous sets for f are required of order type λ, for every λ < κ.

The existence of any of these kinds of Ramsey cardinal is sufficient to prove the existence of 0#, or indeed that every set with rank less than κ has a sharp.

Every measurable cardinal is a Ramsey cardinal, and every Ramsey cardinal is a Rowbottom cardinal.

A property intermediate in strength between Ramseyness and measurability is existence of a κ-complete normal non-principal ideal I on κ such that for every AI and for every function

f: [κ] → {0, 1}

there is a set BA not in I that is homogeneous for f. This is strictly stronger than κ being ineffably Ramsey.

The existence of a Ramsey cardinal implies the existence of the zero sharp cardinal and this in turn implies the falsity of the Axiom of Constructibility of Kurt Gödel.