Christian Tix
Self-adjointness and spectral properties of a pseudo-relativistic
Hamiltonian due to Brown and Ravenhall
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ABSTRACT.    Some properties of a pseudo-relativistic Hamiltonian describing a
  one electron atom - an appropriately projected Dirac operator with
  Coulomb potential - proposed by Brown and Ravenhall are proven.
  Self-adjointness is investigated and the explicit form of the
  Friedrichs extension is given. The behavior of the operator near the
  critical coupling constant $Z_c$ is studied and the essential 
  spectrum is determined in the case of $Z=Z_c$.