Evaluation metrics for regression http://blog.csdn.net/u013704227/article/details/77604500
NP problem explained http://www.matrix67.com/blog/archives/105
Gradient descent: https://www.cnblogs.com/pinard/p/5970503.html
Gradient Descent: http://ruder.io/optimizing-gradient-descent/
Saddle point: https://blog.csdn.net/BVL10101111/article/details/78051939