Past exam of the model theory and Gödel's incompleteness theorems course of the University of Oxford / 2008 / B / 8 / i / Solution

The Ryll-Nardzewski theorem says that for a complete theory $T$ in a countable first-order language with infinite first-order models, the following are equivalent: $T$ is ${\aleph }_0$-categorical; for every positive integer $n$, the set $S_n(T)$ of complete parameter-free $n$-types is finite; every such finite-arity complete type is isolated by a logical formula. The countable-language hypothesis and the requirement for every finite arity are important.

The omitting types theorem says that, for a first-order consistent complete theory in a countable first-order language, a countable family of nonprincipal partial types in finite tuples of logical variables can all be omitted in a countable first-order model. Here nonprincipal means that no logical formula first-order consistent with the first-order theory implies every logical formula of the partial type. Omission means that no tuple in the constructed first-order model satisfies all logical formulas of that partial type. The complete one-type version is the special case of one partial type in one logical variable.