Union of Saturated Models in Superstable Abstract Elementary Classes

In this paper we prove: Theorem 1. Let $\mathcal{K}$ be an abstract elementary class which satisfies the joint embedding and amalgamation properties. Suppose $\lambda>\mu\geq LS(\mathcal{K})$ and $\theta$ is a limit ordinal $<\lambda^+$. If $\mathcal{K}$ is $\mu$ superstable and $\mu^+$-superstable and satisfies $\mu^+$-symmetry, then for any increasing sequence $\langle M_i\mid i<\theta\rangle$ of $\mu^+$-saturated models of cardinality $\lambda$, the model $\bigcup_{i<\theta}M_i$ is $\mu^+$-saturated.