1. On majorization, factorization, and range inclusion of operators on Hilbert space;Douglas, R. G.;Proc. Amer. Math. Soc.,1966
2. R. G. Douglas, Addendum for “Majorization, factorization, and range inclusion" (unpublished).
3. Investigation concerning positive definite continued fractions;MacNerney, J. S.;Duke Math. J.,1959
4. Two-sided division in the ring of operators;Šmul′jan, Ju. L.;Mat. Zametki,1967