General results on the eigenvalues of operators with gaps, arising from   both ends of the gaps. Application to Dirac operators
  This paper is concerned with {an extension and reinterpretation} of previous results on the variational characterization of eigenvalues in gaps of the essential spectrum of self-adjoint operators. {We state} two general abstract results on the existence of eigenvalues in the gap and a continuation principle. Then, these results are applied to Dirac operators in order to characterize simultaneously eigenvalues corresponding to electronic and positronic bound states. 
Jean Dolbeault (CEREMADE), Maria J. Esteban (CEREMADE), Eric s\'er\'e   (CEREMADE)