EN
The elementary theory of ⟨α;×⟩, where α is an ordinal and × denotes ordinal multiplication, is decidable if and only if $α < ω^{ω}$. Moreover if $|_{r}$ and $|_{l}$ respectively denote the right- and left-hand divisibility relation, we show that Th $⟨ω^{ω^{ξ}};|_{r}⟩$ and Th $⟨ω^{ξ};|_{l}⟩$ are decidable for every ordinal ξ. Further related definability results are also presented.