TITLE: Probability and Entropy
QUESTION [5 upvotes]: According to the Wikipedia article on conditional entropy, $\sum p(x,y)\log p(x)=\sum p(x)\log p(x)$. Can someone please explain how?

REPLY [3 votes]: In the article $p(x,y)$ is summed (integrated) over all possible values of y for a fixed value of x.
$\sum_{y\in\mathcal Y} p(x,y)= p(x)$