1. D. Betten, T. Grundhöfer, H. Hähl, R. Löwen, H. Salzmann, and M. Stroppel.Compact Projective Planes. deGruyter, Berlin, 1995
2. J. Hilgert and K.H. Hofmann. Old and new on SL(2).Manuscr. Math. 64, 17–52 (1985)
3. J. Hilgert, K.H. Hofmann, and J.D. Lawson.Lie Groups, Convex Cones and Semigroups. Oxford University Press, 1989
4. Lecture Notes in Math-ematics;J. Hilgert,1993
5. D. Mittenhuber. New sufficient criteria for the globality of Lie wedges.Seminar Sophus Lie 3, 35–46 (1993)