  We show that the von Neumann's algorithm of reduction (i.e. the algorithm of
calculating the density matrix of the observable subsystem from the density
matrix of the closed quantum system) corresponds to the special approximation
at which the unobservable subsystem is supposed to be in the steady state of
minimum information (infinite temperature). We formulate the generalized
algorithm of reduction that includes as limiting cases the von Neumann's
reduction and the self-congruent correlated reduction most corresponding to the
quantum nondemolition measurement. We demonstrate the correlation in dynamics
of subsystems with exactly soluble models of quantum optics: 1) about the
dynamics of a pair of interacting two-level atoms, and 2) about the dynamics of
a two-level atom interacting with a single-mode resonant field.
