1. Division of Science and Mathematics, New York University Abu Dhabi, P.O. Box 129188, United Arab Emirates
2. Equa Simulation AB, Råsundavägen 100, Solna, Sweden
3. Department of Mathematics, Stockholm University, SE-106 91 Stockholm, Sweden
4. Independent University of Moscow, Bolshoj Vlasievsky per, dom 11, RU-119 002 Moscow, Russia