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kl散度意义：
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In the context of machine learning,
is often called the information gain achieved if Q is used instead of P. -
This reflects the asymmetry in Bayesian inference, which starts from a prior Q and updates to the posterior P.
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公式：
证明：https://stats.stackexchange.com/questions/7440/kl-divergence-between-two-univariate-gaussians