### Torsion-free abelian groups are consistently $a\Delta\frac{1}{2}$-complete

by Shelah and Ulrich. [ShUl:1141]

Let TFAG be the theory of torsion-free abelian groups. We
show that if there is no countable transitive model of ZF^- +
kappa (omega) exists, then mbox {TFAG} is a Delta^1_2-complete;
in particular, this is consistent with ZFC . We define the
alpha-ary Schroder- Bernstein property, and show that
mbox {TFAG} fails the alpha-ary Schroder-Bernstein property
for every alpha < kappa (omega) . We leave open whether or not
mbox {TFAG} can have the kappa (omega)-ary Schroder-Bernstein
property; if it did, then it would not be a Delta^1_2-complete, and
hence not Borel complete.

Back to the list of publications