1. Department of Mathematics , Donghua University , Shanghai 201620 , P. R. China
2. Department of Mathematics , Donghua University , Shanghai 201620; and Jining Senior Vocational School, Jining 272031 , P. R. China
3. Department of Mathematics , Donghua University , Shanghai 201620; and Nanjing University, Nanjing 210093 , P. R. China