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

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 $n\le \omega$, ${=}^{+n}$ 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 $n=1$.