Minimal Energy on Unicyclic Graphs

Abstract:

      For a simple graph $G$, the energy $E(G)$ is defined as the sum of the absolute values of all eigenvalues of its adjacency matrix. Let $\mathscr{U}_{n}$ denote the set of all connected unicyclic graphs with order $n$, and $\mathscr{U}^{r}_{n}=\{G\in\mathscr{U}_{n}|\,d(x)=r$ for any vertex $x\in V(C_{\ell})\}$, where $r\geq 2$ and $C_{\ell}$ is the unique cycle in $G$. Every unicyclic graph in $\mathscr{U}^{r}_{n}$ is said to be a cycle-$r$-regular graph. In this paper, we completely characterize that $C_{9}^{3}(2,2,2)\circ S_{n-8}$ is the unique graph having minimal energy in $\mathscr{U}_{n}^{4}$. Moreover, the graph with minimal energy is uniquely determined in $\mathscr{U}_{n}^{r}$ for $r=3,4$.