[1] Univ Putra Malaysia, Fac Sci, Dept Math & Stat, Upm Serdang 43400, Selangor, Malaysia
[2] Univ Putra Malaysia, Inst Math Res, Upm Serdang 43400, Selangor, Malaysia
[3] Univ Teknikal Malaysia Melaka, Fak Teknol & Kejuruteraan Mekanikal, Durian Tunggal 76100, Melaka, Malaysia
[4] Babes Bolyai Univ, Dept Math, R-400084 Cluj Napoca, Romania
[5] Acad Romanian Scientists, 3 Ilfov St, Bucharest 050044, Romania