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$\newcommand{\E}{\mathrm{E}}$ I don't understand why Baum-Welch algorithm is an instantiation of EM algorithm.

Indeed, why computing $\alpha_t(i)$ and $\beta_t(i)$ corresponds to Expectation step. Expectation step corresponds to compute expectation over the latent variable of log-likelihood of observed variable given the parameter: $\E_{Z|X,\theta^{t-1}}[L(X|\theta)]$.

I don't understand the link.

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The EM algorithm computes the expectation of the complete data log-likelihood, not the observed data log-likelihood, conditional on the observed data and current estimate of the parameters. The complete data log-likelihood can be written with indicators for the latent data. The expectation of an indicator is its probability. The expectation of the product of two indicators is the joint probability.

$\gamma_i(t) = E[ I(X_t=i) \vert Y, \theta] = P(X_t=i \vert Y, \theta)$

$\xi_i(t) = E[ I(X_t=i)I(X_{t+1}=j) \vert Y, \theta] = P(X_t=i, X_{t+1}=j \vert Y, \theta)$

Computing each of these conditional probabilities is the E-step, but computing them requires computing each of them requires computing all the $\alpha$'s and $\beta$'s.

See the Gaussian mixture example for a simpler illustration of how latent variables can be written as indicators in the complete data likelihood http://en.wikipedia.org/wiki/Expectation%E2%80%93maximization_algorithm

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