In this paper, we consider the question of definability of types in non‐stable theories. In order to do this we introduce a notion of a relatively stable theory: a theory is stable up to Δ if any Δ‐type over a model has few extensions up to complete types. We prove that an n‐type over a model of a theory that is stable up to Δ is definable if and only if its Δ‐part is definable.

中文翻译：

---

关于类型的可定义性和相对稳定性

在本文中，我们考虑了非稳定理论中类型的可定义性问题。为了做到这一点，我们引入一个相对稳定理论的概念：如果模型上的任何Δ类型几乎没有扩展到完整类型，则该理论对Δ稳定。我们证明，在且仅当其Δ部分是可定义的时，在模型上稳定到Δ的n型是可定义的。