The QED beta-function from global solutions to Dyson-Schwinger equations
  We discuss the structure of beta functions as determined by the recursive nature of Dyson--Schwinger equations turned into an analysis of ordinary differential equations, with particular emphasis given to quantum electrodynamics. In particular we determine when a separatrix for solutions to such ODEs exists and clarify the existence of Landau poles beyond perturbation theory. Both are determined in terms of explicit conditions on the asymptotics for the growth of skeleton graphs. 
Guillaume van Baalen, Dirk Kreimer, David Uminsky, and Karen Yeats