The Myhill isomorphism theorem does not generalize much
The Myhill isomorphism is a variant of the Cantor-Bernstein theorem. It states that, from two injections that reduce two subsets of $\mathbb{N}$ to one another, there exists a bijection $\mathbb{N} \to \mathbb{N}$ that preserves them. This theorem can be proven constructively. We investigate to which extent the theorem can be extended to infinite sets other than $\mathbb{N}$. We show that, assuming Markov's principle, the theorem can be extended to the conatural numbers $\mathbb{N}_{\infty}$ 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 $2 \times \mathbb{N}_{\infty}$, $\mathbb{N} + \mathbb{N}_{\infty}$, $\mathbb{N} \times \mathbb{N}_{\infty}$, $\mathbb{N}_{\infty}^2$, $2^{\mathbb{N}}$ or $\mathbb{N}^{\mathbb{N}}$.