An analogue of the Sommerfeld radiation condition for the Dirac operator
  A simple radiation condition at infinity for time-harmonic massive Dirac spinors is proposed. This condition allows an analogue of the Cauchy integral formula in unbounded domains for null-solutions of the Dirac equation to be proved. The result is obtained with the aid of methods of quaternionic analysis. 
Vladislav V. Kravchenko, Raul Castillo P