On the evaluation of the norm of an integral operator associated with   the stability of one-electron atoms
  The norm of an integral operator occurring in the partial wave decomposition of an operator B introduced by Brown and Ravenhall in a model for relativistic one-electron atoms is determined. The result implies that B is non-negative and has no eigenvalue at 0 when the nuclear charge does not exceed a specified critical value. 
V.I.Burenkov and W.D.Evans