Mathematical logic seminar - Feb 19 2019

Time: 3:30pm - 4:30 pm

Room: Wean Hall 8220

Speaker:     Assaf Shani    
Department of Mathematics
UCLA

Title: Borel reducibility and symmetric models

Abstract:

We develop a correspondence between the study of Borel equivalence relations induced by closed subgroups of S, and the study of symmetric models of set theory without choice, and apply it to prove a conjecture of Hjorth-Kechris-Louveau (1998). For example, we show that the equivalence relation ≅*ω+1,0 is strictly below ≅*ω+1 in Borel reducibility. By results of Hjorth-Kechris-Louveau, ≅*ω+1 corresponds to Σ0ω+1 actions of S, while ≅*ω+1,0 corresponds to Σ0ω+1 actions of "well behaved" closed subgroups of S, for example abelian groups. For these proofs we analyze the models Mn, n<ω, developed by Monro (1973), and extend his construction past ω, through all countable ordinals. This answers a question of Karagila (2016).