On Mean-Field Theory of Quantum Phase Transition in Granular   Superconductors
  In previous work on quantum phase transition in granular superconductors, where mean-field theory was used, an assumption was made that the order parameter as a function of the mean field is a convex up function. Though this is not always the case in phase transitions, this assumption must be verified, what is done in this article. 
M.V.Simkin (Brown University)