The construction of nonequilibrium entropy is related to a certain regularization of the
von Neumann entropy corresponding to the kinetic stage of evolution. It is shown that in
the third approximation of perturbation theory undetermined constants appear in the entropy.
In this connection, the construction of the nonequilibrium entropy is related to the
question of determining products of generalized functions with coincident singularities in
the Bogolyubov–Parasyuk sense.