July 2018
Intermediate to advanced
310 pages
8h 21m
Chinese
Content preview from 深度学习入门 : 基于Python的理论与实现
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5.2 链式法则
129
5.2.3
链式法则和计算图
现在我们尝试将式（5.4）的链式法则的计算用计算图表示出来。如果用
“
**
2
”节点表示平方运算的话，则计算图如图5
-
7 所示。
+
**
2
y
x
zt
图5-7 式（5.4）的计算图：沿着与正方向相反的方向，乘上局部导数后传递
如图所示，计算图的反向传播从右到左传播信号。反向传播的计算顺序
是，先将节点的输入信号乘以节点的局部导数（偏导数），然后再传递给下一
个节点。比如，反向传播时，“
**
2
”节点的输入是 ，将其乘以局部导数 （因
为正向传播时输入是t、输出是z，所以这个节点的局部导数是
），然后传
递给下一个节点。另外，图5
-
7 中反向传播最开始的信号
在前面的数学
式中没有出现，这是因为
，所以在刚才的式子中被省略了。
图 5
-
7 中需要注意的是最左边的反向传播的结果。根据链式法则，
成立，对应“z 关于x 的导数”。也就是说，反向传播
是基于链式法则的。
把 式（5.3）的结果代入到图5
-
7 中，结果如图5
-
8 所示，
的结果为
2
(
x
+
y
)
。
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