### Non-hyperfinite countable equivalence relations

This will be a talk at the CUNY Models of Peano Arithmetic seminar on Wednesday, March 15th 2017. (Added later: the talk overspilled into the next two weeks of the MoPA seminar.)

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.