作者机构:[1]School of Mathematical Sciences (and the Key Laboratory on Mathematics and ComplexSystem), Beijing Normal University, Beijing 100875, China, [2]Center of Mathematical Sciences, Zhejia-ng-University, Hangzhou 310027, China
相关基金:Acknowledgements The authors would like to show their respect to the referees for their suggestions, especially on the form of the conclusion 'rad(N) ≥rad(X) 〉 π/2' in Main Theorem (in the original version of the paper, the conclusion is 'rad(N) 〉 π/2'). This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 11001015, 11171025).