The behavior of higher proof theory I: Case $Σ^1_2$

Abstract: The current ordinal analysis provides the proof-theoretic ordinal of a theory, which calibrates the robustness of the $\Pi^1_1$-consequences of the theory. We can ask whether there is an ordinal characteristic capturing more complex consequences, and it turns out that we can define the $\Sigma^1_2$-proof-theoretic ordinal capturing the robustness of the $\Sigma^1_2$-consequences of a theory. In this paper, we study the behavior of $\Sigma^1_2$-proof-theoretic ordinal, and it turns out that $\Sigma^1_2$-proof-theoretic ordinal also follows an analogue of Walsh's characterization of proof-theoretic ordinal.