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Infinitary stability theory

We introduce a new device in the study of abstract elementary classes (AECs): Galois Morleyization, which consists in expanding the models of the class with a relation for every Galois type of length less than a fixed cardinal $κ$. We show: $\mathbf{Theorem}$ (The semantic-syntactic correspondence) An AEC $K$ is fully $(<κ)$-tame and type short if and only if Galois types are syntactic in the Galois Morleyization. This exhibits a correspondence between AECs and the syntactic framework of stability theory inside a model. We use the correspondence to make progress on the stability theory of tame and type short AECs. The main theorems are: $\mathbf{Theorem}$ Let $K$ be a $\text{LS}(K)$-tame AEC with amalgamation. The following are equivalent: * $K$ is Galois stable in some $λ\ge \text{LS}(K)$. * $K$ does not have the order property (defined in terms of Galois types). * There exist cardinals $μ$ and $λ_0$ with $μ\le λ_0 < \beth_{(2^{\text{LS}(K)})^+}$ such that $K$ is Galois stable in any $λ\ge λ_0$ with $λ= λ^{<μ}$. $\mathbf{Theorem}$ Let $K$ be a fully $(<κ)$-tame and type short AEC with amalgamation, $κ= \beth_κ > \text{LS} (K)$. If $K$ is Galois stable, then the class of $κ$-Galois