Hu Fei was born in 1977. He is the Ph. D. candidate in School of Electrical Engineering and Automation and a lecture in Department of Mathematics, Tianjin University. His research interests include system modeling and reliability analysis. E-mail: hfzdzy@126.com Xu Genqi was born in 1959. Now he is a professor in Department of Mathematics, Tianjin University. He received his Ph.D. degree from the System Science In-stitute of Chinese Academy of Sciences. His research interests are spectral theory of linear operator, system stability and reliability analysis, etc. Ma Lixia was born in 1978. She is the Ph.D. candidate in School of Electrical Engineering and Automation and a lecture in Department of Mathematics, Tianjin University. Her research interest is in system stability analysis.