We compute the virtual O(\alpha_s^3 n_f^2) corrections to the heavy quark vector current correlator in terms of expansions in the external momentum and as an exact numerical solution. As a byproduct, the available high-energy expansion at the three-loop level is extended.
@article{arxiv.0712.2762,
title = {Double fermionic contributions to the heavy-quark vacuum polarization},
author = {M. Czakon and T. Schutzmeier},
journal= {arXiv preprint arXiv:0712.2762},
year = {2008}
}