We present rigorous upper and lower bounds for the zero-momentum gluon
propagator D(0) of Yang-Mills theories in terms of the average value of the
gluon field. This allows us to perform a controlled extrapolation of lattice
data to infinite volume, showing that the infrared limit of the Landau-gauge
gluon propagator in SU(2) gauge theory is finite and nonzero in three and in
four space-time dimensions. In the two-dimensional case we find D(0) = 0, in
agreement with Ref. [1]. We suggest an explanation for these results. We note
that our discussion is general, although we only apply our analysis to pure
gauge theory in Landau gauge. Simulations have been performed on the IBM
supercomputer at the University of Sao Paulo.Comment: 4 pages, 3 figures, 1 tabl