A universal behaviour is predicted for ghost and gluon propagators in the infrared.
All lattice data, for different color numbers (and even quark content for the ghosts)
collapse on the same universal curve by a scaling of units and by
adding an integration constant. The universal behaviour is shown to be a signature of a
one-loop approximation and emerges naturally by any modified one-loop approximation that
is capabale of predicting a massive gluon, provided that the Lagrangian is not modified and that the gluon mass arises from the loops in the expansion.
For instance, a massive expansion, with massive gluon propagators in the loops, is shown to
predict universal analytical functions for the derivatives of the inverse dressing functions that do not depend on any parameter or color number. All lattice data are shown to collapse on the anlytical curves.