11.2.1 定义

式\((11.5)\)的推导过程：
令\(Z=P(X|Y)\sim P_1(Z)\)，则

\[E_{Y\sim P(Y)}[-\log P(X|Y)]=E_{Z\sim P_1(Z)}[-\log Z] \]

\[\geq -\log E[Z]=-\log \int P(Y)P(X|Y)dY=-\log X \]