  A nonperturbative renormalization of the phi^4 model is considered. First we
integrate out only a single pair of conjugated modes with wave vectors +/- q.
Then we are looking for the RG equation which would describe the transformation
of the Hamiltonian under the integration over a shell Lambda - d Lambda < k <
Lambda, where d Lambda -> 0. We show that the known Wegner--Houghton equation
is consistent with the assumption of a simple superposition of the integration
results for +/- q. The renormalized action can be expanded in powers of the
phi^4 coupling constant u in the high temperature phase at u -> 0. We compare
the expansion coefficients with those exactly calculated by the diagrammatic
perturbative method, and find some inconsistency. It causes a question in which
sense the Wegner-Houghton equation is really exact.
