Affiliation: | (1) Fachbereich Mathematik und Informatik, Universit?t Duisburg-Essen, Campus Essen, DE-45117 Essen, Germany;(2) Department of Pure and Applied Mathematics, Graduate School of Information Science and Technology, Osaka University, Toyonaka, Osaka 560-0043, Japan;(3) Department of Mathematical Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan |