# Nordhaus-Gaddum type results for the Harary index of graphs

Document Type: Research Paper

Authors

1 Beijing Normal Unviersity

2 Qinghai Normal Unviersity

3 Qinghai Normal University

Abstract

The \emph{Harary index} $H(G)$ of a connected graph $G$ is defined as $H(G)=\sum_{u,v\in V(G)}\frac{1}{d_G(u,v)}$ where $d_G(u,v)$
is the distance between vertices $u$ and $v$ of $G$. The
Steiner distance in a graph, introduced by Chartrand et al. in
1989, is a natural generalization of the concept of classical graph
distance. For a connected graph $G$ of order at least $2$ and
$S\subseteq V(G)$, the \emph{Steiner distance} $d_G(S)$ of the
vertices of $S$ is the minimum size of a connected subgraph whose
vertex set contains $S$. Recently, Furtula, Gutman, and Katani'{c} introduced the concept
of Steiner Harary index and gave its chemical applications. The \emph{$k$-center Steiner Harary index} $SH_k(G)$ of $G$ is
defined by $SH_k(G)=\sum_{S\subseteq V(G),|S|=k}\frac{1}{d_G(S)}$.
In this paper, we get the sharp upper and lower
bounds for $SH_k(G)+SH_k(\overline{G})$ and $SH_k(G)\cdot SH_k(\overline{G})$, valid for any connected graph $G$ whose
complement $\overline {G}$ is also connected.

Keywords

Main Subjects

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