  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.
