On the Levinson theorem for Dirac operators

For the Dirac equation with potential V(r) obeying ∫∞ 0(1+r)‖V(r)‖d r<∞ we prove a relativistic version of Levinson’s theorem that relates the number of bound states in the spectral gap [−m,m] to the variation of an appropriate phase along the continuous part of the spectrum. In the process, the asymptotic properties of the Jost function as E→±m are analyzed in detail. The connection with the nonrelativistic version of Levinson’s theorem is also established.