ON THE CANONICAL RAMSEY THEOREM OF ERDŐS AND RADO AND RAMSEY ULTRAFILTERS

Abstract

We give a characterizations of Ramsey ultrafilters on ω in terms of functions \(f:{{\omega }^{n}} \to \omega \) and their ultrafilter extensions. To do this, we prove that for any partition \(\mathcal{P}\) of \({{[\omega ]}^{n}}\) there is a finite partition \(\mathcal{Q}\) of \({{[\omega ]}^{{2n}}}\) such that any set \(X \subseteq \omega \) that is homogeneous for \(\mathcal{Q}\) is a finite union of sets that are canonical for \(\mathcal{P}\).