一個簡單的圖形示例
舉一個簡單的例子,我們使用以下圖形:
下面是其numpy鄰接矩陣表示形式。
A = np.matrix([
[0, 1, 0, 0],
[0, 0, 1, 1],
[0, 1, 0, 0],
[1, 0, 1, 0]],
dtype=float
)
接下來,根據其索引為每個節點生成2個整數特征,便于以后手動確認矩陣計算。
In [3]: X = np.matrix([
[i, -i]
for i in range(A.shape[0])
], dtype=float)
X
Out[3]: matrix([
[ 0., 0.],
[ 1., -1.],
[ 2., -2.],
[ 3., -3.]
])
應用傳播規則
好吧!現在,我們有了一個圖形,其鄰接矩陣A和一組輸入要素X。讓我們看看應用傳播規則時會發生什么:
In [6]: A * X
Out[6]: matrix([
[ 1., -1.],
[ 5., -5.],
[ 1., -1.],
[ 2., -2.]]
發生了什么?現在,每個節點(每行)的表示形式都是其鄰居特征的總和!換句話說,圖卷積層將每個節點表示為其鄰居的集合。注意,在這種情況下,如果存在從v到n的邊,則節點n是節點v的鄰居。
問題
你可能已經發現了問題:
節點的匯總表示不包括其自身的功能!
該表示是鄰居節點特征的集合,因此只有具有自環的節點才會在集合中包括自己的特征。度數較大的節點的特征將具有較大的值,而度數較小的節點將具有較小的值。這可能會導致梯度消失或爆炸,但對于隨機梯度下降算法(通常用于訓練此類網絡并且對每個輸入特征的比例(或值的范圍))也存在問題。在下文中,我將分別討論每個問題。
添加自環
為了解決個問題,可以簡單地向每個節點添加一個自環。實際上,這是通過在應用傳播規則之前將單位矩陣I與鄰接矩陣A相加來完成的。
In [4]: I = np.matrix(np.eye(A.shape[0]))
IOut[4]: matrix([
[1., 0., 0., 0.],
[0., 1., 0., 0.],
[0., 0., 1., 0.],
[0., 0., 0., 1.]
])
In [8]: A_hat = A + I
A_hat * X
Out[8]: matrix([
[ 1., -1.],
[ 6., -6.],
[ 3., -3.],
[ 5., -5.]])
由于該節點現在是其自身的鄰居,因此在總結其鄰居的特征時會包括該節點自己的特征!
規范特征表示
通過將鄰接矩陣A與D的逆矩陣相乘來變換鄰接矩陣A,可以按節點度對特征表示進行歸一化。因此,我們簡化的傳播規則如下所示:
f(X,A)=D??AX
讓我們看看發生了什么。我們首先計算度矩陣。
In [9]: D = np.array(np.sum(A, axis=0))[0]
D = np.matrix(np.diag(D))
D
Out[9]: matrix([
[1., 0., 0., 0.],
[0., 2., 0., 0.],
[0., 0., 2., 0.],
[0., 0., 0., 1.]
])
在應用規則之前,讓我們看看對鄰接矩陣進行轉換后會發生什么。
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