1. Doctoral Program of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Jawa Barat 45363, Indonesia
2. Department of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Jawa Barat 45363, Indonesia
3. School of Dental Sciences, Universiti Sains Malaysia, Health Campus, Kubang Kerian 16150, Malaysia