数学物理学报 ›› 2000, Vol. 20 ›› Issue (4): 461-467.
(南开大学数学所 天津 300071)
(浙江大学应用数学系 杭州玉泉 310027)
(Nankai Institute of Mathematics, Nankai University, Tianjin 300071, China)
(Department of Mathematics, Zhejiang University, Hangzhou 310027, Zhejiang, China)