A B C Z. Naturforsch.68a,531 – 538 (2013)doi:10.5560/ZNA.2013-0031On the Kirchhoff Index of GraphsDepartment of Mathematics, Sungkyunkwan University, Suwon 440-746, Republic of KoreaReceived December 20, 2012 / revised March 15, 2013 / published online May 22, 2013Reprint requests to: K. C. D.; E-mail: kinkardas2003@googlemail.comLetGbe a connected graph of ordernwith Laplacian eigenvaluesμ_{1}≥μ_{2}≥…≥μ_{n−1}>μ_{n}= 0. The Kirchhoff index ofGis defined as Kf = Kf(G) =n∑_{k=1}^{n−1}1/μ_{k}.In this paper. we give lower and upper bounds on Kf of graphs in terms on

n, number of edges, maximum degree, and number of spanning trees. Moreover, we present lower and upper bounds on the Nordhaus–Gaddum-type result for the Kirchhoff index.Key words:Graph Spectrum; Laplacian Spectrum (of Graph); Kirchhoff Index; Nordhaus–Gaddum-Type.Mathematics Subject Classification 2000:05C50; 15A18