1. Department of Mathematics and Statistics, University of Massachusetts, Amherst, Massachusetts 01003-4515, USA
2. Departamento de Física, MSI-Nucleus on Advanced Optics, and Center for Optics and Photonics (CEFOP), Facultad de Ciencias, Universidad de Chile, Santiago, Chile
3. Department of Mathematics, University of the Aegean, Karlovassi, 83200 Samos, Greece