Characterizations of bipartite and Eulerian partial duals of orientable hypermaps

Abstract

We first rewrite the Chmutov and Vignes-Tourneret's three-permutation formula as an explicit hyperedge-partial-duality formula in the two-permutation model, and show that in this model partial duality acts exactly by preserving the support and length of every hyperedge while reversing the $α$-cycles corresponding to the selected hyperedges. Next, using the Cori and Hetyei's construction of the medial map, we define for each hyperedge subset $E'\subseteq E(H)$ a black/white smoothing state $S_{E'}$, and prove rigorously that the state circles of $S_{E'}$ are in bijection with the vertices of the partial dual $H^{E'}$. Consequently, $H^{E'}$ is Eulerian if and only if every state circle has even length. On this basis we prove the following two main theorems: \[ \begin{aligned} H^{E'}\text{ is Eulerian} &\Longleftrightarrow \exists\text{ a crossing-total direction $Ω$ of }M(H) \\ &\hspace{3.35em}\text{such that } E'=D(Ω)\cup T',\quad T'\subseteq T(Ω), \end{aligned} \] \[ \begin{aligned} H^{E'}\text{ is bipartite} &\Longleftrightarrow \exists\text{ an all-crossing direction $Φ$ of }M(H) \\ &\hspace{3.35em}\text{such that } E'=C(Φ). \end{aligned} \] Here $D(Ω)$, $T(Ω)$ and $C(Φ)$ denote, respectively, the sets of all $d$-type, $t$-type and $c$-type hyperedges. Unlike the ribbon-graph case, the hypermap setting exhibits a genuine new obstruction: if some hyperedge-partial dual is bipartite, then every hyperedge of the original hypermap must have even length.