梯度下降算法 https://www.jianshu.com/p/c7e642877b0e
极大似然估计详解: https://blog.youkuaiyun.com/class_brick/article/details/79724660 https://blog.youkuaiyun.com/zengxiantao1994/article/details/72787849
SVM https://blog.youkuaiyun.com/crcr/article/details/60346084 PCA https://blog.youkuaiyun.com/hustqb/article/details/78394058