The authors wish to thank the two anonymous reviewers and the handling editor Dr. Tenreiro Machado for their comments and suggestions, which led to a great improvement to the presentation of this work. Research is supported by National Natural Science Foundation of China (Grant Nos. 31501227, 61603283, 61672395), Social Science Foundation of Hunan Province (Grant No. 15YBA204) and the Fundamental Research Funds for the Central Universities (Grant Nos. 2014-Ia-038 and 2016-IA-001).