Affiliation: | 1. Department of Natural Sciences, Ibaraki National College of Technology, 866 Nakane, Hitachinaka-shi, Ibaraki-ken, 192-0397, Japan 2. Department of Mathematics and Information Sciences, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Ohsawa, Hachioji, 192-0397, Japan
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