1Institut für Theoretische Teilchenphysik,
Universität Karlsruhe, D-76128 Karlsruhe, Germany
2Max-Planck-Institut für Physik, Werner-Heisenberg-Institut, D-80805 Munich, Germany
Received: 20 October 1997
In this article we evaluate mass corrections up to to the three-loop polarization function induced by an axial-vector current. Special emphasis is put on the evaluation of the singlet diagram which is absent in the vector case. As a physical application corrections to the production of top quarks at future e+e- colliders are considered. It is demonstrated that for center of mass energies GeV the inclusion of the first seven terms into the cross section leads to a reliable description.