1. Fundamental Physics Laboratory, Physics of Complex System Group, Department of Physics, Faculty of Science, University of Douala 1 , Box 24 157 Douala, Cameroon
2. Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Telegraphenberg 2 , P.O. Box 601203, 14473 Potsdam, Germany
3. Complex Systems and Theoretical Biology Group and Laboratory of Research on Advanced Materials and Non-linear Science (LaRAMaNS), Department of Physics, Faculty of Science, University of Buea 3 , P.O. Box 63, Buea, Cameroon
4. Department of Physics, Humboldt University 4 , 12489 Berlin, Germany