Four-loop moments of the heavy quark vacuum polarization function in perturbative QCD K.G. Chetyrkin, J.H. Kühn and C. Sturm Eur. Phys. J. C, 48 1 (2006) 107-110 Published online: 29 August 2006 DOI: 10.1140/epjc/s2006-02610-y