Levinson theorem for Dirac particles in two dimensions

The Levinson theorem for nonrelativistic quantum mechanics in two spatial dimensions is generalized to Dirac particles moving in a central field. The theorem relates the total number of bound states with angular momentum $j$ ($j=\pm 1/2, \pm 3/2, ... $), $n_j$, to the phase shifts $\eta_j(\pm E_k)$ of scattering states at zero momentum as follows: $\eta_j(\mu)+\eta_j(-\mu)= n_j\pi$.