1. Department of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Jatinangor, Sumedang 45363, Indonesia
2. Department of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada, North Sekip, Yogyakarta 55281, Indonesia
3. Department of Statistics, Faculty of Sciences and Mathematics, Universitas Diponegoro, Tembalang, Semarang 50275, Indonesia
4. Doctoral Program of Mathematics, Faculty of Mathematics and Natural Sciences, Universitas Padjadjaran, Jatinangor, Sumedang 45363, Indonesia