1. Vrije Universiteit Amsterdam, Department of Mathematics, Faculteit der Exacte Wetenschappen, De Boelelaan 1081a, 1081 HV Amsterdam, The Netherlands, and MathNeuro Team, Inria Centre at Université Côte d’Azur, 2004 route des Lucioles-Boîte Postale 93 06902, Sophia Antipolis Cedex, France.