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Published online by Cambridge University Press: 29 June 2023
We calculate the possible Scott ranks of countable models of Peano arithmetic. We show that no non-standard model can have Scott rank less than $\omega $ and that non-standard models of true arithmetic must have Scott rank greater than
$\omega $. Other than that there are no restrictions. By giving a reduction via
$\Delta ^{\mathrm {in}}_{1}$ bi-interpretability from the class of linear orderings to the canonical structural
$\omega $-jump of models of an arbitrary completion T of
$\mathrm {PA}$ we show that every countable ordinal
$\alpha>\omega $ is realized as the Scott rank of a model of T.