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# 行列の微分(2)

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(概要)
『ゼロから作るDeep Learning』読書会＠高円寺(15)に参加してきた。
https://koenjidl.connpass.com/event/69408/
この書籍で、下記(1)式他の導出が省略されていた。

あいまいな点がのこるので、要素に分解して導出した。

(方針)

スカラ値による微分に、置き換えて解く。

(求める式)

(1)
\begin{equation*}
\frac{\partial L}{\partial W} =X^{T} ・\frac{\partial L}{\partial Y}
\end{equation*}


(変数)

\begin{gather*}
(2)Y=( y_{1} ,y_{2} ,y_{3})\\
(3)X=( x_{1} ,x_{2})\\
(4)W=\begin{pmatrix}
w_{11} & w_{21} & w_{31}\\
w_{12} & w_{22} & w_{32}
\end{pmatrix}
\end{gather*}


(前準備)

(5)
\begin{equation*}
XW=\left( x_{1} w_{11} +x_{2} w_{12} ,\ x_{1} w_{21} +x_{2} w_{22} ,\ x_{1} w_{31} +x_{2} w_{32}\right)
\end{equation*}

(6)
\begin{equation*}
y_{k} =\sum _{m} x_{m} ・w_{km} +b_{k}
\end{equation*}


(式展開)

\begin{gather*}
\frac{\partial L}{\partial w_{11}} =\sum _{i}\frac{\partial L}{\partial y_{i}} ・\frac{\partial y_{i}}{\partial w_{11}}\\
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =\frac{\partial L}{\partial y_{1}} ・x_{1} +\frac{\partial L}{\partial y_{2}} ・0+\frac{\partial L}{\partial y_{3}} ・0\\
=\frac{\partial L}{\partial y_{1}} ・x_{1}\\
\frac{\partial L}{\partial w_{ij}} =\sum _{k}\frac{\partial L}{\partial y_{k}} ・\frac{\partial y_{k}}{\partial w_{ij}}\\
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =\sum _{k}\frac{\partial L}{\partial y_{k}} ・\left(\frac{\partial }{\partial w_{ij}}\left(\sum _{m} x_{m} ・w_{km} +b_{k}\right)\right)\\
=\frac{\partial L}{\partial y_{i}} ・x_{j}\\
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{\partial L}{\partial W} =\begin{pmatrix}
\frac{\partial L}{\partial y_{1}} ・x_{1} & \frac{\partial L}{\partial y_{2}} ・x_{1} & \frac{\partial L}{\partial y_{3}} ・x_{1}\\
\frac{\partial L}{\partial y_{1}} ・x_{2} & \frac{\partial L}{\partial y_{2}} ・x_{2} & \frac{\partial L}{\partial y_{3}} ・x_{2}
\end{pmatrix}\\
\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ =\begin{pmatrix}
x_{1}\\
x_{2}
\end{pmatrix} ・\begin{pmatrix}
\frac{\partial L}{\partial y_{1}} & \frac{\partial L}{\partial y_{2}} & \frac{\partial L}{\partial y_{3}}
\end{pmatrix}\\
\ =X^{T} ・\frac{\partial L}{\partial Y}
\end{gather*}

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