1. School of Technology and Management Polytechnic Institute of Leiria Leiria P‐2411‐901 Portugal
2. CIDMA ‐ Center for Research and Development in Mathematics and Application, Department of Mathematics University of Aveiro, University Campus of Santiago Aveiro 3810‐193 Portugal
3. Department of Mathematics‐Physics‐Chemistry Beuth Technical University of Applied Sciences Berlin Berlin 13353 Germany