  Improvements of various methods to compute the sign function of the hermitian
Wilson-Dirac matrix within the overlap operator are presented. An optimal
partial fraction expansion (PFE) based on a theorem of Zolotarev is given.
Benchmarks show that this PFE together with removal of converged systems within
a multi-shift CG appears to approximate the sign function times a vector most
efficiently. A posteriori error bounds are given.
