  We give a nonrigorous derivation of the nonlinear Boltzmann equation from the
Schrodinger evolution of interacting fermions. The argument is based mainly on
the assumption that a quasifree initial state satisfies a property called
restricted quasifreeness in the weak coupling limit at any later time. By
definition, a state is called restricted quasifree if the four-point and the
eight-point functions of the state factorize in the same manner as in a
quasifree state.
