Dynamical virial relations and invalidity of the Boltzmann kinetic   equation
  A sequence of exact relations is found which connect one- and many-particle time-dependent distribution functions of low-density gas with their derivatives in respect to mean density. It is shown that, at least in the context of spatially non-uniform gas evolutions, these relations forbid the "molecular chaos propagation" and imply inapplicability of the Boltzmann kinetic equation even under the Boltzmann-Grad limit and regardless of degree of the non-uniformity. 
Yu. E. Kuzovlev
cond-mat.stat-mech