1. Departamento de Matemática Aplicada, Universidad Complutense de Madrid, Madrid, Spain. E-mail: arrieta@mat.ucm.es
2. Institut für Mathematik. Universität Zürich, Zürich, Switzertland. E-mail: maria.lopez@math.uzh.ch
3. Basque Center for Applied Mathematics, Bizkaia Technology Park, Derio, Basque Country, Spain. E-mail: zuazua@bcamath.org
4. Ikerbasque – Basque Foundation for Science, Bilbao, Basque Country, Spain