1. Two loops in the absolute plane;Capodaglio;Mitt. Math. Ges. Hamburg,2004
2. Recent developments on absolute geometries and algebraization by K-loops;Karzel;Discrete Math.,1999
3. Loops, reflection structures and graphs with parallelism;Karzel;Result. Math.,2002
4. From involution sets, graphs and loops to loop-nearrings;Karzel,2005
5. H. Karzel, K. Sörensen, D. Windelberg, Einführung in die Geometrie, Uni-Taschenbücher 184, Göttingen, 1973, ISBN 3-525-03406-7.