LEVINSON THEOREM FOR THE DIRAC-EQUATION

Levinson's theorem for the Dirac equation is known in the form of a sum of positive and negative energy phase shifts at zero momentum related to the total number of bound states. In this Letter we prove a stronger version of Levinson's theorem valid for positive and negative energy phase shifts separately. The surprising result is that, in general, the phase shifts for each sign of the energy do not give the number of bound states with the same sign of the energy (in units of pi), but instead are related to the number of bound states of a certain Schrodinger equation, which coincides with the Dirac equation at zero momentum.