文章链接:http://mp.weixin.qq.com/s?__biz=MzIxMjAzNDY5Mg==&mid=2650790918&idx=1&sn=4fad7df685991d3dbbd2469fe8c5aedf&scene=21#wechat_redirect 原文: https://hit-scir.gitbooks.io/neural-networks-and-deep-learning-zh_cn/content/chap6/c6s1.html