中国科学院数学与系统科学研究院期刊网
On the Index of the Normal Subgroups of Fundamental Groups of Graph
Wei ZHOU(1); Wu Jie SHI(2)
Acta Mathematica Sinica, Chinese Series . 2007, (4): 797 -800 .  DOI: 10.12386/A2007sxxb0092