1. Institute for Mathematical Research, Universiti Putra Malaysia (UPM), Selangor, 43400 Serdang, Malaysia
2. School of Mathematical Sciences, Faculty of Science & Technology, Universiti Kebangsaan Malaysia (UKM), Selangor, 43600 Bangi, Malaysia
3. Department of Mathematics, Babeş-Bolyai University, 400084 Cluj-Napoca, Romania