We study the strength of axioms needed to prove various results related to automata on infinite words and B\"uchi's theorem on the decidability of the MSO theory of $(N, {\le})$. We prove that the following are equivalent over the weak second-order arithmetic theory $RCA_0$: (1) the induction scheme for $\Sigma^0_2$ formulae of arithmetic, (2) a variant of Ramsey's Theorem for pairs restricted to so-called additive colourings, (3) B\"uchi's complementation theorem for nondeterministic automata on infinite words, (4) the decidability of the depth-$n$ fragment of the MSO theory of $(N, {\le})$, for each $n \ge 5$. Moreover, each of (1)-(4) implies McNaughton's determinisation theorem for automata on infinite words, as well as the "bounded-width" version of K\"onig's Lemma, often used in proofs of McNaughton's theorem.

@article{paperbot792,
  title = {The logical strength of B\"uchi's decidability theorem},
  author = {Leszek Kołodziejczyk and Henryk Michalewski and Cécilia Pradic and Michał Skrzypczak},
  journal = {Logical Methods in Computer Science},
  volume = {Volume 15, Issue 2},
  year = {2019},
  doi = {10.23638/lmcs-15(2:16)2019}
}