The atom-bond connectivity (ABC) index of a graph G,is defined as the sum of the weights (du+dv-2/dudv)1/2 of all edges uv of G,where du denotes the degree of a vertex u in G.The ABC index provides a good model for the stabihty of hnear and branched alkanes as well as the strain energy of cycloalkanes.The sharp lower bounds on ABC index of chemical unicyclic graphs is obtained.%图G的原子键连通性指数的定义如下:ABC(G)=∑uv∈E(G)(du+dv-2/dudv)1/2.其中du、dv分别表示图G的边uv的2个端点u、v的度数.ABC指数已被证实为研究烷烃的稳定性以及环烷烃的应变能提供了一个很好的模型.讨论了n阶化学单圈图,给出了其ABC指数的可达的下界及其相应的极图.
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