说明：  如何计算矩阵数据的协方差矩阵。还有在什么情况下只能计算均值、方差，而不能计算协方差，什么情下，只能计算协方差，但不能计算协方差矩阵。
(How to calculate the matrix data covariance matrix. And under what circumstances can only calculation mean, variance, and not calculation covariance, what feeling, can only calculation covariance, but not calculation covariance matrix.)