1. 自己對自己做covariance就variance
2. pandas裡面Covariance的函數有四種，基本上如下：
(Documentation裡關鍵字Covariance就找的到了，嗚)
[pandas.core.groupby.SeriesGroupBy.cov](https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.SeriesGroupBy.cov.html#pandas.core.groupby.SeriesGroupBy.cov)
[pandas.Series.cov](https://pandas.pydata.org/docs/reference/api/pandas.Series.cov.html#pandas.Series.cov)
[pandas.core.groupby.DataFrameGroupBy.cov](https://pandas.pydata.org/docs/reference/api/pandas.core.groupby.DataFrameGroupBy.cov.html#pandas.core.groupby.DataFrameGroupBy.cov)
[pandas.DataFrame.cov](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.cov.html#pandas.DataFrame.cov)

如果要回傳兩軸相同的Coviarance，應使用後兩者