Minimal Posets Realizing \texorpdfstring{$\mathbb{Z}_2 \times \mathbb{Z}_4$} as Automorphism Group

Abstract

We prove $β(\mathbb{Z}_2 \times \mathbb{Z}_4) = 14$, where $β(G)$ denotes the minimum cardinality $|P|$ among finite posets $P$ with $\Aut(P) \cong G$. The lower bound is established by a complete case analysis of orbit decompositions of $P$ under faithful $G$-actions, organized by the largest orbit size. The upper bound is realized by an explicit $14$-element poset whose automorphism group is computed by a height-function argument together with a rigidity analysis of its covering relations.