1 Department of Applied Mathematics, NUI Galway, Galway, Ireland; Department of Mathematics, Thu Dau Mot University, Binh Duong, Vietnam. Electronic address: vinhmq@tdmu.edu.vn.
2 MACSI, Department of Mathematics and Statistics, University of Limerick, Limerick, Ireland.
3 Department of Applied Mathematics, NUI Galway, Galway, Ireland.