A Note on Computable Embeddings for Ordinals and Their Reverses

Abstract

We continue the study of computable embeddings for pairs of structures, i.e. for classes containing precisely two non-isomorphic structures. Surprisingly, even for some pairs of simple linear orders, computable embeddings induce a non-trivial degree structure. Our main result shows that although $\{ω\cdot 2, ω^\star \cdot 2\}$ is computably embeddable in $\{ω^2, {(ω^2)}^\star\}$, the class $\{ω\cdot k,ω^\star \cdot k\}$ is \emph{not} computably embeddable in $\{ω^2, {(ω^2)}^\star\}$ for any natural number $k \geq 3$.