Affiliation: | Department of Mathematics, National University of Singapore 10 Kent Ridge Crescent, 0511, Republic of Singapore Department of Mathematics and Science, National Taipei Teacher's College 134, Ho Ping E. Road, Sec. 2, Taipei 10659, Taiwan, R.O.C. St. John & St. Mary's Institute of Technology Tamsui, Taipei, Taiwan, R.O.C. |