1 Aizu Research Cluster for Medical Engineering and Informatics, Center for Advanced Information Science and Technology, The University of Aizu, Aizuwakamatsu, Fukushima 965-8580, Japan. Electronic address: tdpham@u-aizu.ac.jp.
2 Department of Mathematics and Computer Science, University of Münster, Einsteinstrasse 62, D-48149 Münster, Germany. Electronic address: xjiang@uni-muenster.de.