所用的拟合模型有sigmoid和tanh的线性组合，sigmoid，tanh，如果前一个拟合模型收敛不了就跳到下一个模型。
感谢丁香园提供的世界各国确诊数据，github地址 https://github.com/BlankerL/DXY-COVID-19-Data 。
A study on confirmed case number of nCOVID-19
This repository includes simple curve fits by country using the daily confirmed cases number.
Three functions are used to fit curves: arctan, sigmoid, and a linear combination of arctan and sigmoid.
Data comes from DingXiangYuan public data, which is available in https://github.com/BlankerL/DXY-COVID-19-Data
Fitted results can be found in the folder Fitted_Curve