文章链接:http://mp.weixin.qq.com/s?__biz=MzIxMjAzNDY5Mg==&mid=2650790603&idx=1&sn=51a2e75c994fdfbf6d54aa71fe8391c4&scene=21#wechat_redirect 原文: https://hit-scir.gitbooks.io/neural-networks-and-deep-learning-zh_cn/content/chap3/c3s8.html