1. Blumenthal, L. M.: Boolean geometry I. Rend. Circ. Mat. Palermo (2)1, 1?18 (1952).
2. Gopalarao, N. R.: Vector spaces over a regular ring. Math. Ann.167, 280?291 (1966).
3. Jagannadham, P. V.: Linear transformations in a Boolean vector space. Math. Ann.167, 240?247 (1966).
4. Penning, C. J.: Minimal duplicator rings. Koninkl. Nederl. Akademie van Wetenschappen, Amsterdam, ser. A,66, 3, 295?312 (1963).
5. Pitcher, E., andM. F. Smiley: Transitivities of betweenness. Trans. Amer. Math. Soc.52, 95?114 (1942).