f-Sum Rule and Unconventional Spectral Weight Transfer in Graphene
  We derive and analyze the f-sum rule for a two-dimensional (2D) system of interacting electrons whose behavior is described by the Dirac equation. We apply the sum rule to analyze the spectral weight transfer in graphene within different approximations discussed in the literature. We find that the sum rule is generically dominated by inter-band transitions while other excitations produce sub-leading behavior. The f-sum rule provides strong constraints for theories of interacting electrons in graphene. 
J. Sabio, J. Nilsson and A. H. Castro Neto