arXiv subjects
Publications and source records attributed to Miaomiao Han.
The \textit{square} of a graph $G$, denoted by $G^2$, is obtained from $G$ by adding an edge to connect every pair of vertices with a common neighbor in $G$. In this paper we prove that for every planar graph $G$ with maximum degree at most $5$, $G^2$ admits a proper vertex coloring using at most $17$ colors, which improves the upper bound $18$ recently obtained by Hou, Jin, Miao, and Zhao.
Let $S,T$ be two distinct finite Abelian groups with $|S|=|T|$. A fundamental theorem of Tutte shows that a graph admits a nowhere-zero $S$-flow if and only if it admits a nowhere-zero $T$-flow. Jaeger, Linial, Payan and Tarsi in 1992 introduced group connectivity as an extension of flow theory, and they asked whether such a relation holds for group connectivity analogy. It was negatively answered by Hušek, Mohelníková and Šámal in 2017 for graphs with edge-connectivity 2 for the groups $S=\mathbb{Z}_4$ and $T=\mathbb{Z}_2^2$. In this paper, we extend their results to $3$-edge-connected graphs (including both cubic and general graphs), which answers open problems proposed by Hušek, Mohelníková and Šámal(2017) and Lai, Li, Shao and Zhan(2011). Combining some previous results, this characterizes all the equivalence of group connectivity under $3$-edge-connectivity, showing that every $3$-edge-connected $S$-connected graph is $T$-connected if and only if $\{S,T\}\neq \{\mathbb{Z}_4,\mathbb{Z}_2^2\}$.
Given a zero-sum function $β: V(G) \rightarrow \mathbb{Z}_3$ with $\sum_{v\in V(G)}β(v)=0$, an orientation $D$ of $G$ with $d^+_D(v)-d^-_D(v)= β(v)$ in $\mathbb{Z}_3$ for every vertex $v\in V(G)$ is called a $β$-orientation. A graph $G$ is $\mathbb{Z}_3$-connected if $G$ admits a $β$- orientation for every zero-sum function $β$. Jaeger et al. conjectured that every $5$-edge-connected graph is $\mathbb{Z}_3$-connected. A graph is $\langle\mathbb{Z}_3\rangle$-extendable at vertex $v$ if any pre-orientation at $v$ can be extended to a $β$-orientation of $G$ for any zero-sum function $β$. We observe that if every $5$-edge-connected essentially $6$-edge-connected graph is $\langle\mathbb{Z}_3\rangle$-extendable at any degree five vertex, then the above mentioned conjecture by Jaeger et al. holds as well. Furthermore, applying the partial flow extension method of Thomassen and of Lovász et al., we prove that every graph with at least 4 edge-disjoint spanning trees is $\mathbb{Z}_3$-connected. Consequently, every $5$-edge-connected essentially $23$-edge-connected graph is $\langle\mathbb{Z}_3\rangle$-extendable at degree five vertex.
The stable charge states of vacancies in the solar cell absorber material Cu2ZnSnS4 are investigated using Kohn-Sham (KS) defect-induced single particle levels analysis by concerning the screened Coulomb hybrid functional. We found out that the Cu, Zn and S vacancies (denoted by VCu, VZn, VS) do not induce single particle defect levels in the vicinity of the band gap thus each of them has only one stable charge state corresponding to the fully occupied valence band VCu1-, VZn2- and VS0, respectively (and therefore cannot account for any defect transition energy levels). The Sn vacancy (VSn) has three stable charge states VSn2-, VSn3- and VSn4-, which may account for two charge transition energy levels. By comparing with previous charge transition energy levels studies, our results indicate that the examination of stable charge states is a necessary and important step which should be done before charge transition energy levels calculations.