Acknowledgements

This work was supported by Natural Science Foundation of China grants (11461021, 11261013), Nature Science Foundation of Guangxi (2015GXNSFAA139011), Research Foundation of Hezhou University (No.HZUBS201302), and Guangxi Education Department Key Laboratory of Symbolic Computation and Engineering Data Processing.

MR(2000) Subject Classification: 34C05, 37C07
