Relativistic spin operator and Dirac equation
  We give a direct link between description of Dirac particles in the abstract framework of unitary representation of the Poincar\'e group and description with the help of the Dirac equation. In this context we discuss in detail the spin operator for a relativistic Dirac particle. We show also that the spin operator used in quantum field theory for spin $s=1/2$ corresponds to the Foldy-Woutheysen mean-spin operator. 
Pawe{\l} Caban and Jakub Rembieli\'nski and Marta W{\l}odarczyk   (University of Lodz)
math-ph math.MP quant-ph