  We perform a Chapman-Enskog expansion of the Boltzmann equation keeping up to
quadratic contributions. We obtain a generalized nonlinear Kubo formula, and a
set of integral equations which resum ladder and extended ladder diagrams. We
show that these two equations have exactly the same structure, and thus provide
a diagrammatic interpretation of the Chapman-Enskog expansion of the Boltzmann
equation, up to quadratic order.
