文章链接：http://mp.weixin.qq.com/s?__biz=MzIxMjAzNDY5Mg==&mid=2650790968&idx=1&sn=8495ef94523d53618ebc8ec517525d68&chksm=8f4749d3b830c0c5c41e0d3ab01d274dd0b579b459a07c9db5681bdc2984a3671f9e3fc27c24&scene=21#wechat_redirect

原文: https://hit-scir.gitbooks.io/neural-networks-and-deep-learning-zh_cn/content/chap6/c6s4.html