Hagendorf Orders

Abstract

Call a linear order type $φ$ Hagendorf if it shares two properties with additively indecomposable ordinal numbers without being one itself: $φ$ is strictly indecomposable to the right and whenever $ψ< φ$, then $ψ$ can be embedded into a proper initial segment of $φ$. In the 1970s, J. Hagendorf asked whether such types exist. F. Galvin observed that they must be uncountable, and soon thereafter, J. Larson proved that they cannot be scattered. We provide a simplification of her argument that might give additional insight into the $σ$-scattered case. We then show that consistently Hagendorf types exist and that this holds in many models of set theory, for instance under $\mathsf{BA}$ or $\mathsf{MA}_{\aleph_1}$. In fact, their non-existence has large cardinal strength. Furthermore, we construct real Hagendorf types from $\diamondsuit$ and from $\mathsf{PFA}$. This marks the first progress on this interesting problem since its description by Larson four dozen years ago.