1. G. F. Birkenmeier and H. Heatherly, Operation inducing systems, Algebra Universalis, 24 (1987), 137?148.
2. G. F. Birkenmeier and H. Heatherly, Medial rings and an associated radical, Czech. Math. J., 40 (1990), 258?283.
3. N. Divinsky, Rings and Radicals, Univ. Toronto Press, 1965.
4. J. Je?ek, T. Kepka, P. N?mec, Distributive Groupoids, Rozpravy ?SAV, ?ada mat. a p?i?. v?d 91/3 (1981).
5. T. Kepka, Varieties of left distributive semigroups, Acta Univ. Carolinae Math. et Phys., 25 (1984), 3?18.