Borel reducibility and symmetric models
We develop a correspondence between Borel equivalence relations induced by closed subgroups of S∞ and 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,<ω in Borel reducibility. By results of Hjorth-Kechris-Louveau, ≅∗ω+1,<ω bounds the complexity of Σ0ω+1 actions of S∞, while ≅∗ω+1,0 bounds the complexity of Σ0ω+1 actions of ``well behaved'' closed subgroups of S∞, such as abelian groups. The notions mentioned above will be defined in the talk, and I will also survey the results of Hjorth-Kechris-Louveau.