Rowbottom cardinal
# Rowbottom cardinal

In set theory, a Rowbottom cardinal, introduced by Rowbottom (1971), is a certain kind of large cardinal number.

An uncountable cardinal number $\kappa$ is said to be $\lambda$- Rowbottom if for every function f: $[κ]^{<ω}$ → λ (where λ < κ) there is a set H of order type $\kappa$ that is quasi-homogeneous for f, i.e., for every n, the f-image of the set of n-element subsets of H has < $\lambda$ elements. $\kappa$ is Rowbottom if it is $\omega _{1}$ - Rowbottom.

Every Ramsey cardinal is Rowbottom, and every Rowbottom cardinal is Jónsson. By a theorem of Kleinberg, the theories ZFC + “there is a Rowbottom cardinal” and ZFC + “there is a Jónsson cardinal” are equiconsistent.

In general, Rowbottom cardinals need not be large cardinals in the usual sense: Rowbottom cardinals could be singular. It is an open question whether ZFC + “$\aleph _{\omega }$ is Rowbottom” is consistent. If it is, it has much higher consistency strength than the existence of a Rowbottom cardinal. The axiom of determinacy does imply that $\aleph _{\omega }$ is Rowbottom (but contradicts the axiom of choice).

