Extremal k-apex Trees for Randic Index

The Randic (connectivity) index is one of the most successful molecular descriptors in structure-property and structure-activity relationships studies. J. Gao found the sharp upper bound for the Randic index of apex trees. In this paper, we proved that k-apex trees are not regular graphs for k\ge2 and proposed a sharp upper bound for the Randic index of k-apex trees for k>1.