回归的评价指标 http://blog.csdn.net/u013704227/article/details/77604500
NP问题解释 http://www.matrix67.com/blog/archives/105
梯度下降:https://www.cnblogs.com/pinard/p/5970503.html
梯度下降:http://ruder.io/optimizing-gradient-descent/
鞍点:https://blog.csdn.net/BVL10101111/article/details/78051939