1. a Mathematical Sciences, Brunel University , Uxbridge , UB8 3PH , UK
2. b Department of Mathematics , University College London , London , WC1E 6BT , UK
3. c Department of Mathematics , Imperial College London , London , SW7 2BZ , UK
4. d Instituto Nacional de Matimática Pura e Aplicada , Estrada Dona Castorina 110, Jardim Botânico, Rio de Janeiro , 22460-320 , Brazil
5. e Imperial College Business School , London , SW7 2BZ , UK