1. Null systems in projective space;Baer, Reinhold;Bull. Amer. Math. Soc.,1945
2. 2. E. Hecke, Theorie der algebraischen Zahlen, Leipzig, 1923.
3. 3. H. Liebmann, Synthetische Geometrie, Leipzig and Berlin, 1934.
4. Bolyai-Lobachevsky planes with finite lines;Topel, Bernard J.;Rep. Math. Colloquium (2),1944
5. 5. O. Veblen and J. W. Young, Projective geometry, vol. 1.