Non-hyperfinite countable equivalence relations

This is a series of three talks at the CUNY MoPA seminar, on 15, 22, and 29 March 2017.

I will present an argument of the Slaman–Steel theorem that the equivalence relation $E_\infty$ is not hyperfinite. Time permitting, I will explain a potential connection between this argument and the classification problem for models of arithmetic.