Abstract
We give a new definition of Scott rank motivated by our main theorem: For every countable structure A and ordinal α < ω1, we have that: every automorphism orbit is Σ α -definable without parameters if and only if A has a Π α+1 Scott sentence, if and only if A is uniformly boldface ∆α-categorical. As a corollary, we show that a structure is computably categorical on a cone if and only if it is the model of a countably categorical Σ 3 sentence.
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