Scott sentences and a problem of Vaught for mono-uary algebras
For a cardinal κ and a model M of cardinality κ let No(M) denote the number of nonisomorphic models of cardinality κ which are -equivalent to M. We prove that for κ a weakly compact cardinal, the question of the possible values of No(M) for models M of cardinality κ is equivalent to the question of the possible numbers of equivalence classes of equivalence relations which are Σ¹₁-definable over . By [SV] it is possible to have a generic extension where the possible numbers of equivalence classes...
We study the -theory of sequences of dual groups and give a complete classification of the -elementary classes by finding simple invariants for them. We show that nonstandard models exist.