1. Department of Environmental and Prevention Sciences, University of Ferrara, Ferrara, Italy
2. Maxwell Institute and Department of Mathematics, Heriot-Watt University, Edinburgh, UK
3. Department of Mathematics and Computer Science, University of Ferrara, Ferrara, Italy
4. Department of Mathematics, University of Pavia, Pavia, Italy
5. IMATI, Institute for Applied Mathematics and Information Technologies "Enrico Magenes", Pavia, Italy