1. Department of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Sumedang 45363, Indonesia
2. Department of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Nusa Cendana, Kupang 85001, Indonesia
3. Department of Mathematics, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, Madhya Pradesh, India
4. Faculty of Engineering, University of Miyazaki, Miyazaki 889-2192, Japan