文章链接:http://mp.weixin.qq.com/s?__biz=MzIxMjAzNDY5Mg==&mid=2650790900&idx=1&sn=b3a6d644ae6340754f7f5148416a59c1&scene=21#wechat_redirect 原文: https://hit-scir.gitbooks.io/neural-networks-and-deep-learning-zh_cn/content/chap6/c6s0.html