School of Mathematics, Zunyi Normal College, Guizhou 563006, P.R.China;
School of Mathematics, Zunyi Normal College, Guizhou 563006, P.R.China;
School of Mathematics, Zunyi Normal College, Guizhou 563006, P.R.China;
School of Mathematics, Zunyi Normal College, Guizhou 563006, P.R.China;