The sound reflection hierarchy classification conjecture

For nNn\in\mathbb{N}, let 0(n)0^{(n)} denote the nnth Turing jump, let BΣ1B\Sigma_1 be the stated base theory, and let Σn+1\Sigma_{n+1}-soundness have its usual meaning.

Sound reflection hierarchy classification conjecture. For every nNn\in\mathbb{N}, both of the following hold: (1) there exists a 0(n)0^{(n)}-recursive sequence (Tn)n<ω(T_n)_{n<\omega} of Σn+1\Sigma_{n+1}-sound recursively enumerable extensions of BΣ1B\Sigma_1 such that, for each nn, TnT_n proves the Σn+1\Sigma_{n+1}-soundness of Tn+1T_{n+1}; and (2) there is no 0(n)0^{(n)}-recursive sequence T(Tn)n<ωT(T_n)_{n<\omega} of Σn+1\Sigma_{n+1}-sound recursively enumerable extensions of BΣ1B\Sigma_1 such that, for each nn, TnT_n proves the Σn+1\Sigma_{n+1}-soundness of Tn+1T_{n+1}.

The source presents this as a hoped-for classification associated with a question about descending sequences in reflection hierarchies. The parser marks it open, and the source gives no resolution.

Sources & referencesView supporting material

Primary source

James Walsh, “On the hierarchy of natural theories”, arXiv:2106.05794 (2025).

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