On the coefficients of the independence polynomial of graphs

Abstract

An independent set of a graph G is a set of pairwise non-adjacent vertices. Let $i_k = i_k(G)$ be the number of independent sets of cardinality k of G. The independence polynomial $I(G, x)=\sum _{k\geqslant 0}i_k(G)x^k$ defined first by Gutman and Harary has been the focus of considerable research recently, whereas $i(G)=I(G, 1)$ is called the Merrifield–Simmons index of G. In this paper, we first proved that among all trees of order n,  the kth coefficient $i_k$ is smallest when the tree is a path, and is largest for star. Moreover, the graph among all trees of order n with diameter at least d whose all coefficients of I(G, x) are largest is identified. Then we identify the graphs among the n-vertex unicyclic graphs (resp. n-vertex connected graphs with clique number $\omega$) which simultaneously minimize all coefficients of I(G, x), whereas the opposite problems of simultaneously maximizing all coefficients of I(G, x) among these two classes of graphs are also solved respectively. At last we characterize the graph among all the n-vertex connected graph with chromatic number $\chi$ (resp. vertex connectivity $\kappa$) which simultaneously minimize all coefficients of I(G, x). Our results may deduce some known results on Merrifield–Simmons index of graphs.