5-矩阵的迹

• https://blog.csdn.net/quintind/article/details/78452471
• https://blog.csdn.net/realitycss/article/details/79939254
• https://www.zybang.com/question/a4e817f4ae284f972a0c601d669bcfe0.html
• https://www.zhihu.com/question/20533117
• https://blog.csdn.net/qq_39742013/article/details/82116970
  协方差矩阵
• https://www.cnblogs.com/terencezhou/p/6235974.html
• https://blog.csdn.net/chenxin0620/article/details/52920211
  协方差的物理意义
• https://blog.csdn.net/sinat_33588424/article/details/79695983