I am looking for a derivation of the following formula(adsbygoogle = window.adsbygoogle || []).push({});

$$

\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?

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# A Kubos formula for viscosity

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