抄録
In this talk I discuss the analytic structure of the fermion propagator, within the context of the Dyson-Schwinger equation. I show that the standard approximations(the ladder approximation and beyond) lead to complex singularities of the fermion propagator, instead of the expected mass singularities in the time-like region. This might be connected with confinement(in QCD), but in four-dimensional QED my conclusion is that this an artifact of the approximation. However, sofar there is no approximation scheme known in which these singularities are located in the time-like region.
