The quark propagator in presence of an arbitrary gluon field is calculated gauge and Lorentz covariantly order by order in terms of powers of gluon field and its derivatives. The result is independent of path connecting ends of propagator and leading order result coincides with the exact propagator in the trivial case of vanishing gluon field.
@article{arxiv.hep-ph/0302023,
title = {Gauge and Lorentz Covariant Quark Propagator in an Arbitrary Gluon Field},
author = {Yong-Hong An and Hua Yang and Qing Wang},
journal= {arXiv preprint arXiv:hep-ph/0302023},
year = {2009}
}