Acta mathematica scientia,Series A ›› 2000, Vol. 20 ›› Issue (4): 493-498.
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2 Department of Mathematics, Henan University, Kaifeng 475001,P.R.China,
3 CCAST and Institute of Applied Mathematics, Academia Sinica, Beijiang 100080,P.R.China,
4 Department of Mathematics,Zhongshan University,Guangzhou 510275,P.R.China)