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In the simple case of mixture of gaussians(with known variance), we have 2 latent variables $\mu$ and $z$. In the vaiational auto-encoder, we assume that the model is infinite mixture of gaussians. If we assume that the variance is known then we have 2 latent variables $\mu$ and $z$. the encoder the responsible for calculating the variational distribution of $z$ (its mean and variance). and the decoder is responsible for calculating the variational distribution of the $\mu$ (it's mean and variance)
So the variance $\sigma$ that the generative network is outputting Is NOT the variance of $x$
My questions is
Is my viewpoint correct?
I read many articles on the internet that say $P(x|z, w) = \mathcal{N}(\mu(z, w),\sigma(z, w))$ where $\mu(.)$ and $\sigma(.)$ are the generative network with parameters $w$. Isn't that wrong?

PS: I know that training and results are the same in both cases

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In a GMM, $z$ is a latent variable, but the $\mu$'s are a model parameter.

You seem to be trying to draw an analogy between GMMs and VAEs -- it's true you can think of a VAE as an infinite mixture model, where each component has $\mu = f(z; \theta)$ where $f$ is the decoder. Here $\theta$, not $\mu$, are the model parameters.

and the decoder is responsible for calculating the variational distribution of the $\mu$

No -- $\mu$ isn't a latent variable, so there's no need for any variational approximation for it.

Yes, $x|z$ has distribution $\mathcal{N}(f(z;\theta))$ -- this is simply how the VAE model is defined.

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  • $\begingroup$ I read before that we can treat GMM as a fully bayesian model and treat $\mu$ as a latent variable and calculate it using variational inference or we can just treat $\mu$ as a parameter and calculate it using EM. Maybe VAE is a variational EM not just variational inference(meaning that we don't treat all the unknown parameters $\mu$ and $z$ as latent variables). Am I right? $\endgroup$
    – floyd
    Commented Sep 12, 2019 at 7:19
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    $\begingroup$ i'm not familiar with this "fully bayesian" form of GMM, so I can't comment on that. A VAE is not trained with EM. It is very much a variational inference method. $z$ is definitely a latent variable. $\endgroup$
    – shimao
    Commented Sep 12, 2019 at 8:28

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