On distance Laplacian spread and Wiener index of a graph

Abstract

Let $G$ be a simple connected simple graph of order $n$. The distance Laplacian matrix $D^L(G)$ is defined as $D^L(G)=Diag(Tr)-D(G)$, where $Diag(Tr)$ is the diagonal matrix of vertex transmissions and $D(G)$ is the distance matrix of $G$. The eigenvalues of $D^L(G)$ are the distance Laplacian eigenvalues of $G$ and are denoted by ${\partial }_1^L(G),{\partial }_2^L(G),\dots ,{\partial }_n^L(G)$. The \textit{ distance Laplacian spread} $DLS(G)$ of a connected graph $G$ is the difference between largest and second smallest distance Laplacian eigenvalues, that is, ${\partial }_1^L(G)-{\partial }_{n-1}^L(G)$. We obtain bounds for $DLS(G)$ in terms of the Wiener index $W(G)$, order $n$ and the maximum transmission degree $T{r}_{max}(G)$ of $G$ and characterize the extremal graphs. We obtain two lower bounds for $DLS(G)$, the first one in terms of the order, diameter and the Wiener index of the graph, and the second one in terms of the order, maximum degree and the independence number of the graph. For a connected $k-partite$ graph $G$, $k\le n-1$, with $n$ vertices having disconnected complement, we show that $DLS(G)\ge \lfloor \frac{n}{k}\rfloor$ with equality if and only if $G$ is a $completek-partite$ graph having cardinality of each independent class same and $n\equiv 0(\mathrm{mod}k)$.

14 pages