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Published online by Cambridge University Press: 20 August 2020
A set $U \subseteq {\mathbb {R}} \times {\mathbb {R}}$ is universal for countable subsets of
${\mathbb {R}}$ if and only if for all
$x \in {\mathbb {R}}$, the section
$U_x = \{y \in {\mathbb {R}} : U(x,y)\}$ is countable and for all countable sets
$A \subseteq {\mathbb {R}}$, there is an
$x \in {\mathbb {R}}$ so that
$U_x = A$. Define the equivalence relation
$E_U$ on
${\mathbb {R}}$ by
$x_0 \ E_U \ x_1$ if and only if
$U_{x_0} = U_{x_1}$, which is the equivalence of codes for countable sets of reals according to U. The Friedman–Stanley jump,
$=^+$, of the equality relation takes the form
$E_{U^*}$ where
$U^*$ is the most natural Borel set that is universal for countable sets. The main result is that
$=^+$ and
$E_U$ for any U that is Borel and universal for countable sets are equivalent up to Borel bireducibility. For all U that are Borel and universal for countable sets,
$E_U$ is Borel bireducible to
$=^+$. If one assumes a particular instance of
$\mathbf {\Sigma }_3^1$-generic absoluteness, then for all
$U \subseteq {\mathbb {R}} \times {\mathbb {R}}$ that are
$\mathbf {\Sigma }_1^1$ (continuous images of Borel sets) and universal for countable sets, there is a Borel reduction of
$=^+$ into
$E_U$.
The author was supported by NSF grant DMS-1703708.