Kurret
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I am looking for a derivation of the following formula
$$
\eta=\lim_{\omega\rightarrow0} \frac{1}{2\omega}\int dt dx\langle[T_{xy}(t,x),T_{xy}(0,0)]\rangle,
$$
where $T_{xy}$ is a component of the stress-energy tensor. This is claimed in for instance https://arxiv.org/pdf/hep-th/0405231.pdf. There seems to be a derivation in https://arxiv.org/pdf/1207.7021.pdf, but it seems overly complicated and involved extra features. So before I dig into that paper to try to understand it, I would like to ask if someone knows a simple derivation of the above Kubo formula for the viscosity?
$$
\eta=\lim_{\omega\rightarrow0} \frac{1}{2\omega}\int dt dx\langle[T_{xy}(t,x),T_{xy}(0,0)]\rangle,
$$
where $T_{xy}$ is a component of the stress-energy tensor. This is claimed in for instance https://arxiv.org/pdf/hep-th/0405231.pdf. There seems to be a derivation in https://arxiv.org/pdf/1207.7021.pdf, but it seems overly complicated and involved extra features. So before I dig into that paper to try to understand it, I would like to ask if someone knows a simple derivation of the above Kubo formula for the viscosity?