1. Department of Mathematics, School of Digital Technologies, Tallinn University, Narva Rd. 25, Tallinn 10120, Estonia; Professor Departamento de Matemáticas y Física, Universidad Autónoma de Aguascalientes, Avenida Universidad 940, Ciudad Universitaria, Aguascalientes 20131, Mexico
2. Department of Mathematics, Nazarbayev University, Kabanbay–Batyr 53, Astana 010000, Kazakhstan