George Bergman, A note on abelian categories – translating element-chasing proofs, and exact embedding in abelian groups (1974) [pdf, pdf]
George M. Bergman, Skew fields of noncommutative rational functions, after Amitsur. In Séminaire Schützenberger–Lentin–Nivat, Année 1969/1970, No. 16. Paris 1970 EuDML