Classifying invariants for $E_1$: A tail of a generic real

TL;DR

This paper develops a framework for classifying complex equivalence relations like $E_1$, preserving classical intuitions, and analyzes the invariants and complexity of such classifications using a specialized Cohen real extension model.

Contribution

It introduces a new framework for studying classification invariants that respects Borel reducibility and applies it to $E_1$, revealing the nature of its invariants and their bounds.

Findings

$E_1$ can be classified by $$-sequences of $E_0$-classes.

Such classification is impossible if $< ext{add}(})$.

Analysis of a Cohen tail intersection model provides bounds on invariants.

Abstract

Let $E$ be an analytic equivalence relation on a Polish space. We introduce a framework for studying the possible "reasonable" complete classifications and the complexity of possible classifying invariants for $E$, such that: (1) the standard results and intuitions regarding classifications by countable structures are preserved in this framework; (2) this framework respects Borel reducibility; (3) this framework allows for a precise study of the possible invariants of certain equivalence relations which are not classifiable by countable structures, such as $E_1$. In this framework we show that $E_1$ can be classified, with classifying invariants which are $\kappa$-sequences of $E_0$-classes where $\kappa=\mathfrak{b}$, and it cannot be classified in such a manner if $\kappa<\mathbf{add}(\mathcal{B})$. These results depend on analyzing the following sub-model of a Cohen real extension, introduced by Kanovei-Sabok-Zapletal (2013) and Larson-Zapletal (2020). Let $\left<c_n:\,n<\omega\right>$ be a generic sequence of Cohen reals, and define the tail intersection model $$M=\bigcap_{n<\omega}V[\left<c_m:\,m\geq n\right>].$$ An analysis of reals in $M$ will provide lower bounds for the possible invariants for $E_1$. We also extend the characterization of turbulence from Larson-Zapletal (2020) in terms of intersection models.