Logic Seminar

Mark PoorCornell University
A Shelah group in ZFC

Friday, September 15, 2023 - 2:55pm
Malott 205

In a paper from 1980, Shelah constructed a Jonsson group of size $\aleph_1$. Assuming CH, he moreover obtained what is now known as a
"Shelah group" of size $\aleph_1$, i.e., a group of size $\aleph_1$ such that for some integer $N$, the collection of all $N$-sized words over the
alphabet of any given uncountable subset of the group resurrects the whole group. In this talk, we shall present a ZFC construction of a Shelah group at the level of any successor of a regular cardinal. We shall also address the problem of constructing Shelah groups at successors of singulars and
at inaccessibles. This is a joint work with Assaf Rinot.