1. Department of Mathematics, Faculty of Science, Mahidol University, Rama VI, Bangkok 10400, Thailand
2. Center of Excellence in Mathematics, Commission on Higher Education, Bangkok, Thailand
3. National Research Council of Italy, Institute for Biomedical Research and Innovation, Via Ugo La Malfa, 153, 90146 Palermo, Italy
4. National Research Council of Italy, Institute for Systems Analysis and Computer Science “A. Ruberti”, BioMatLab (Biomathematics Laboratory), UCSC Largo A. Gemelli 8, 00168 Rome, Italy