The steps essentially involved in the evaluation of transport coefficients in linear response theory using Kubo formulas are to relate the definingretardedcorrelation function to the correspondingtime-orderedone and to evaluate the latter in the conventional perturbation expansion. Here we evaluate the viscosities of a pion gas carrying out both the steps in thereal-timeformulation. We also obtain the viscous coefficients by solving the relativistic transport equation in the Chapman-Enskog approximation to leading order. An in-mediumππcross-section is used in which spectral modifications are introduced in the propagator of the exchangedρ.
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