| Résume | Given a first order theory or a sentence $\Phi$ of $L_\omega_1,\omega$, we define the class of potential canonical Scott sentences of $\Phi$. Simply by comparing cardinalities, we obtain new results about the Borel complexity of $(Mod(\Phi),\iso)$, the class of countable models of $\Phi$. In particular, we find examples of first order theories T for which Mod(T) is not Borel complete, yet the isomorphism relation on Mod(T) is not Borel.
This is joint work with Douglas Ulrich and Richard Rast. |
