Institution: | a Dept. of Math., Harbin Inst. of Tech., Wei Hai, Shan Dong 264209, PR China b Dept. of Math., Harbin Inst. of Tech., Harbin, Hei Long Jiang 150001, PR China c Dept. of Math. and Mech., Heilongjiang Inst. of Sci. and Tech., Harbin, Hei Long Jiang 150027, PR China |