Extremal values on the harmonic number of trees

Abstract

Let G = (V (G), E(G)) be a simple connected graph. The harmonic number of G, denoted by H(G), is defined as the sum of the weights 2/(d(u) + d(v)) of all edges uv of G, where d(u) denotes the degree of a vertex u in G. In this paper, some extremal problems on the harmonic number of trees are studied. The extremal values on the harmonic number of trees with given graphic parameters, such as pendant number, matching number, domination number and diameter, are determined. The corresponding extremal graphs are characterized, respectively.