1. Department of Mathematics and Informatics, Ibaraki University, Mito 310-8512, Japan
2. Research Center for Ubiquitous MEMS and Micro Engineering, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8564, Japan
3. Department of Engineering Science and Mechanics, Shibaura Institute of Technology, Koto, Tokyo 135-8548, Japan