May 31
Tin Lok Wong, National University of Singapore
Another quantifier-elimination result in arithmetic under negated induction

In a paper published in 1990, Kossak showed that all countable models of $\Sigma_n$ collection where $\Sigma_n$ induction fails have continuum-many automorphisms. We extract from his proof a(nother) quantifier-elimination result. This gives new information about pigeonhole principles and expansions to second-order models. The work is joint with David Belanger, CT Chong, Wei Li, and Yue Yang at the National University of Singapore.