First Author:   Hsiao-Chi Chen National Taipei University hchen@mail.ntpu.edu.tw   Department of Economics, National Taipei University, 151 University Road San-Shia District, 23741, Taiwan   886-2-86741111-ext. 67128(Phone) 886-2-86741111-ext. 67128(Fax) Second Author: Yunshyong Chow Institute of Mathematics, Academia Sinica chow@math.sinic.edu.tw   Institute of Mathematics, Academia Sinica Taipei, 115, Taiwan, ROC.       Third Author: Li-Chau Wu Department of Mathematics, National Taitung University wuli@nttu.edu.tw   Department of Mathematics, National Taitung University Taitung, Taiwan, ROC.