The work of the first author is supported by NSF of China (No. 11571216), Application-Oriented Characterized Disciplines, Double First-Class University Project of Hunan Province (Xiangjiaotong [2018] 469), the Science and Technology Plan Project of Hunan Province (No. 2016TP1020) and the Science and Technology Plan Project of Hengyang City (2017KJ183). The work of the second author is supported by the Mathematical Research Impact Centric Support of the Department of Science and Technology, India (MTR/2017/000367). The authors thank the referee for valuable comments.