No CrossRef data available.
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.