Vertex functions and infrared fixed point in Landau gauge SU(N)   Yang-Mills theory
  The infrared behaviour of vertex functions in an SU(N) Yang-Mills theory in Landau gauge is investigated employing a skeleton expansion of the Dyson-Schwinger equations. The three- and four-gluon vertices become singular if and only if all external momenta vanish while the dressing of the ghost-gluon vertex remains finite in this limit. The running coupling as extracted from either of these vertex functions possesses an infrared fixed point. In general, diagrams including ghost-loops dominate in the infrared over purely gluonic ones. 
Reinhard Alkofer, Christian S. Fischer, Felipe J. Llanes-Estrada