Complexity of Scott Sentences
We give effective versions of some results on Scott sentences. We show that if $\mathcal{A}$ has a computable $Π_α$ Scott sentence, then the orbits of all tuples are defined by formulas that are computable $Σ_β$ for some $β<α$. (This is an effective version of a result of Montalbán.) We show that if a countable structure $\mathcal{A}$ has a computable $Σ_α$ Scott sentence and one that is computable $Π_α$, then it has one that is computable $d$-$Σ_β$ for some $β< α$. (This is an effective version of a result of A. Miller.) We also give an effective version of a result of D. Miller. Using the non-effective results of Montalbán and A. Miller, we show that a finitely generated group has a $d$-$Σ_2$ Scott sentence iff the orbit of some (or every) generating tuple is defined by a $Π_1$ formula. Using our effective results, we show that for a computable finitely generated group, there is a computable $d$-$Σ_2$ Scott sentence iff the orbit of some (every) generating tuple is defined by a computable $Π_1$ formula.