给定具有像<math>\mathcal X</math>的离散随机变量<math>X</math>和具有像<math>\mathcal Y</math>的离散随机变量<math>Y</math>,将给定<math>X</math>的<math>Y</math>的条件熵定义为以<math>p(x)</math>作为权重,对<math>x</math>的每个可能取值得到的<math>H(Y|X=x)</math>的加权和。其表达式如下:<ref name=cover1991>{{cite book|isbn=0-471-06259-6|year=1991|authorlink1=Thomas M. Cover|author1=T. Cover|author2=J. Thomas|title=Elements of Information Theory|url=https://archive.org/details/elementsofinform0000cove|url-access=registration}}</ref>{{rp|15}}
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给定具有像<math>\mathcal X</math>的离散随机变量<math>X</math>和具有像<math>\mathcal Y</math>的离散随机变量<math>Y</math>,将给定<math>X</math>的<math>Y</math>的条件熵定义为以<math>p(x)</math>作为权重,对<math>x</math>的每个可能取值得到的<math>H(Y|X=x)</math>的加权和。其表达式如下:<ref name="cover1991">{{cite book|isbn=0-471-06259-6|year=1991|authorlink1=Thomas M. Cover|author1=T. Cover|author2=J. Thomas|title=Elements of Information Theory|url=https://archive.org/details/elementsofinform0000cove|url-access=registration}}</ref>{{rp|15}}