1. Departamento de Matemáticas, Universidad Carlos III de Madrid, Leganés, Spain
2. Departamento de Matemáticas, Universidad del País Vasco UPV/EHU, Bilbao, Spain
3. Department of Mathematics and Systems Analysis, Aalto University, Aalto, Finland
4. Department of Mathematical Engineering, Université catholique de Louvain, Louvain-la-Neuve, Belgium