Regular Jónsson cardinals

Set theory seminarFriday, December 2, 201610:00 amGC 6417

Shehzad Ahmed

Regular Jónsson cardinals

Ohio University

For an infinite cardinal $\lambda$, we say that $\lambda$ is Jónsson if it satisfies the square bracket partition property $\lambda\to[\lambda]^{<\omega}_\lambda$. This means that, for every coloring $F\colon [\lambda]^{<\omega}\to\lambda$, there is some set $H\in [\lambda]^\lambda$ with the property that $\mathrm{ran}(F\upharpoonright [H]^{<\omega})\subsetneq\lambda$. It is rather well known that the consistency strength of "there is a Jónsson cardinal" lies above "$0^\sharp$ exists", but below the existence of a measurable. However, the question of what sorts of cardinals can be Jónsson has turned out to be rather difficult. The goal of this talk is to sketch a simplified proof of the result (due to Shelah) that if $\lambda$ is the least regular Jónsson cardinal, then $\lambda$ must be $\lambda\times\omega$-Mahlo. The advantage of the proof presented here is that the machinery employed can be easily generalized, and time permitting I would like to discuss how one might attempt to prove that such a cardinals must be greatly Mahlo.

Shehzad Ahmed is a PhD student at Ohio University studying set theory under the supervision of Dr. Todd Eisworth.

Posted by on November 16th, 2016