The finite Friedman-Stanley jumps: generic dichotomies for Borel homomorphisms

TL;DR

This paper establishes a dichotomy for Borel homomorphisms from Friedman-Stanley jumps to classifiable equivalence relations, providing new insights into their reducibility and structural properties using Baire-category techniques.

Contribution

It introduces a novel presentation of Friedman-Stanley jumps enabling the application of Baire-category methods and proves several new results about their reducibility spectrum and regularity.

Findings

$=^{+n}$ is in the spectrum of the meager ideal for $n\,\leq\,\omega$

$=^{+\omega}$ is a regular equivalence relation

Classifiable equivalence relations not reducing $=^{+n}$ are closed under countable products

Abstract

Fix $n=1,2,3,\dots$ or $n=\omega$. We prove a dichotomy for Borel homomorphisms from the $n$-th Friedman-Stanley jump $=^{+n}$ to an equivalence relation $E$ which is classifiable by countable structures: if there is no reduction from $=^{+n}$ to $E$, then in fact all Borel homomorphisms are very far from a reduction. For this we use a different presentation of $=^{+n}$, equivalent up to Borel bi-reducibility, which is susceptible to Baire-category techniques. This dichotomy is seen as a method for proving positive Borel reducibility results from $=^{+n}$. As corollaries we prove: (1) for $n\leq\omega$, $=^{+n}$ is in the spectrum of the meager ideal. This extends a result of Kanovei, Sabok, and Zapletal for $n=1$; (2) $=^{+\omega}$ is a regular equivalence relation. This answers positively a question of Clemens; (3) for $n<\omega$, the equivalence relations, classifiable by countable structures, which do not Borel reduce $=^{+n}$ are closed under countable products. This extends a result of Kanovei, Sabok, and Zapletal for $n=1$.