1. Department of Mathematics, University of Patras, 26500 Patras, Greece
2. Center for Integrable Systems, P. G. Demidov Yaroslavl State University, 150003 Yaroslavl, Russia
3. Department of Mathematics and Didactics of Mathematics, School of Digital Technologies, Tallinn University, Narva Rd. 25, 10120 Tallinn, Estonia
4. Departamento de Matemáticas y Física, Universidad Autónoma de Aguascalientes, Avenida Universidad 940, Ciudad Universitaria, Aguascalientes 20100, Mexico