1. Department of Mathematics, Texas A&M University-Kingsville, Kingsville, TX 78363, USA
2. Department of Mathematics and Systems Engineering, Florida Institute of Technology, Melbourne, FL 32901, USA
3. Faculty of Mathematics and Informatics, Plovdiv University, Tzar Asen 24, 4000 Plovdiv, Bulgaria
4. School of Mathematical and Statistical Sciences, University of Galway, H91 TK33 Galway, Ireland