The substitution lemma for Scott complexity of finite-rank trees

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Let m1m\geq 1. Write Am+1\mathcal{A}_{m+1} for the canonical tree of rank m+1m+1, and let Bj(m):=[m+1:j]B^{(m)}_j:=[m+1:j] be the tree obtained by deleting all but jj of the level-one subtrees of type [m:ω][m:\omega]. Thus Bj(m)≇Am+1B^{(m)}_j\not\cong\mathcal{A}_{m+1}. A root of a [m:ω][m:\omega]-subtree may be relocated to a fresh root of a [m:k][m:k]-subtree, with witnesses carried to matched witnesses. Substitution lemma, two prongs. For every m1m\geq 1, (a) every Π2m1in\Pi^{\mathrm{in}}_{2m-1} formula true in Am+1\mathcal{A}_{m+1} of a tuple consisting of such a root and arbitrary witnesses remains true after relocation for all sufficiently large finite kk; and (b) for every θΠ2min\theta\in\Pi^{\mathrm{in}}_{2m} and every tuple aˉ\bar a satisfying Am+1θ(aˉ)\mathcal{A}_{m+1}\models\theta(\bar a), if jj exceeds the number of [m:ω][m:\omega]-roots named by aˉ\bar a, then Bj(m)θ(aˉ)B^{(m)}_j\models\theta(\bar a) by re-hosting the inner Π2m2in\Pi^{\mathrm{in}}_{2m-2} witnesses inside Bj(m)B^{(m)}_j. Consequently, these two substitution principles should exclude Π2m+1in\Pi^{\mathrm{in}}_{2m+1}, Σ2m+1in\Sigma^{\mathrm{in}}_{2m+1}, and d-Σ2m+1in\mathrm{d}\text{-}\Sigma^{\mathrm{in}}_{2m+1} Scott sentences for Am+1\mathcal{A}_{m+1}, and yield

SSC(Am+1)=Π2m+2in.\operatorname{SSC}(\mathcal{A}_{m+1})=\Pi^{\mathrm{in}}_{2m+2}.

This would provide a purely combinatorial proof of the general Scott-complexity theorem, without computability, and would localize the non-Σ2min\Sigma^{\mathrm{in}}_{2m}-definable orbit at OωmO^m_\omega. The program is stated to remain open for m2m\geq 2.

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Primary source

Mohammad Mahmoud and Mostafa Mirabi, “Scott complexity of trees of finite rank via degrees of categoricity”, arXiv:2606.27151 (2026).

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