1. Department of Civil Engineering, National Institute of Technology, Gunma College, 580 Toriba-cho, Maebashi-shi, Gunma, 371-8530, Japan
2. Department of Mathematics, National Institute of Technology, Gunma College, 580 Toriba-cho, Maebashi-shi, Gunma, 371-8530, Japan
3. Department of Information Science, University of Fukui, 3-9-1, Bunkyo, Fukui-shi, Fukui, 910-8507, Japan