1. Department of Mathematics and Center for Astrophysics, Space Physics and Engineering Research, Baylor University, Waco, TX 76798-7328, USA
2. Department of Mathematics, School of Digital Technologies, Tallinn University, Narva Rd. 25, 10120 Tallinn, Estonia
3. Departamento de Matemáticas y Física, Universidad Autónoma de Aguascalientes, Avenida Universidad 940, Ciudad Universitaria, Aguascalientes, Ags. 20131, Mexico