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\begin{center}
{\Large The Evidence Lower Bound}
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Fran\c cois Fleuret
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\today
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-Given i.i.d training samples $x_1, \dots, x_N$ that follows an unknown
-distribution $\mu_X$, we want to fit a model $p_\theta(x,z)$ to it,
-maximizing
+Given i.i.d training samples $x_1, \dots, x_N$ we want to fit a model
+$p_\theta(x,z)$ to it, maximizing
%
\[
\sum_n \log \, p_\theta(x_n).
$p_\theta(z \mid x_n)$ and $q(z)$, and we may get a worse
$p_\theta(x_n)$ to bring $p_\theta(z \mid x_n)$ closer to $q(z)$.
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+
However, all this analysis is still valid if $q$ is a parameterized
function $q_\alpha(z \mid x_n)$ of $x_n$. In that case, if we optimize
$\theta$ and $\alpha$ to maximize
it maximizes $\log \, p_\theta(x_n)$ and brings $q_\alpha(z \mid
x_n)$ close to $p_\theta(z \mid x_n)$.
-
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