  The finite-temperature renormalization group is formulated via the
Wilson-Kadanoff blocking transformation. Momentum modes and the Matsubara
frequencies are coupled by constraints from a smearing function which plays the
role of an infrared cutoff regulator. Using the scalar lambda phi^4 theory as
an example, we consider four general types of smearing functions and show that,
to zeroth-order in the derivative expansion, they yield qualitatively the same
temperature dependence of the running constants and the same critical exponents
within numerical accuracy.
