1. Dipartimento di Scienze Matematiche DISMA G. L. Lagrange, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
2. School of Mathematics, Statistics and Applied Mathematics, NUI Galway, University Road, Galway, Ireland
3. School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland
4. School of Mathematics, University of Manchester, Oxford Road, Manchester M13 9PL, UK