Find indefinable set that is included in definable set.

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Find $K\subseteq \operatorname{Ass} $ and $ K'\subseteq K$ such that $K$ is definable but $K'$ is not.

Definition (Definable): a set $K$ of assignments is definable if there is a set of formulas A such that $\operatorname{Ass}(A) = K$.

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Hint: $K=\mathit{Ass}$ is definable, so all you need to do is find some non-definable set of assignments.

You need to involve infinitely many propositional variable, because every truth functional of finitely many variables can be defined by a single formula.

How about the set of all possible assignments except the one that makes every propositional variable false?