文章链接:http://mp.weixin.qq.com/s?__biz=MzIxMjAzNDY5Mg==&mid=2650790854&idx=1&sn=6a7d238a0d558f4e0d6ca02e4d0b6d5d&scene=21#wechat_redirect 原文: https://hit-scir.gitbooks.io/neural-networks-and-deep-learning-zh_cn/content/chap5/c5s0.html