1. Department of Mathematics and BGSMath, Universitat Politècnica de Catalunya, 14 08028 Barcelona, Spain;
2. Department of Applied Mathematics, Illinois Institute of Technology, Chicago, Illinois 60616, USA;
3. Institute for Data, Systems, and Society, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4. Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA;