English

The Myhill isomorphism theorem does not generalize much

Logic 2025-07-08 v1 Logic in Computer Science

Abstract

The Myhill isomorphism is a variant of the Cantor-Bernstein theorem. It states that, from two injections that reduces two subsets of N\mathbb{N} to each other, there exists a bijection NN\mathbb{N} \to \mathbb{N} that preserves them. This theorem can be proven constructively. We investigate to which extent the theorem can be extended to other infinite sets other than N\mathbb{N}. We show that, assuming Markov's principle, the theorem can be extended to the conatural numbers N\mathbb{N}_{\infty} provided that we only require that bicomplemented sets are preserved by the bijection. This restriction is essential. Otherwise, the picture is overall negative: among other things, it is impossible to extend that result to either 2×N2 \times \mathbb{N}_{\infty}, N+N\mathbb{N} + \mathbb{N}_{\infty}, N×N\mathbb{N} \times \mathbb{N}_{\infty}, N2\mathbb{N}_{\infty}^2, 2N2^{\mathbb{N}} or NN\mathbb{N}^{\mathbb{N}}.

Keywords

Cite

@article{arxiv.2507.05028,
  title  = {The Myhill isomorphism theorem does not generalize much},
  author = {Cécilia Pradic},
  journal= {arXiv preprint arXiv:2507.05028},
  year   = {2025}
}

Comments

25 pages, 6 figures, draft

R2 v1 2026-07-01T03:49:32.992Z