Lenz 1961, p. 138, but see also Cameron 1991, chapter 3 Lenz, H. (1961), Grundlagen der Elementarmathematik, Berlin: Deutscher Verlag d. Wiss. Cameron, Peter J. (1991), Projective and Polar Spaces, QMW Maths Notes, vol. 13, London: Queen Mary and Westfield College School of Mathematical Sciences, MR1153019
Lenz 1961, p. 138, but see also Cameron 1991, chapter 3 Lenz, H. (1961), Grundlagen der Elementarmathematik, Berlin: Deutscher Verlag d. Wiss. Cameron, Peter J. (1991), Projective and Polar Spaces, QMW Maths Notes, vol. 13, London: Queen Mary and Westfield College School of Mathematical Sciences, MR1153019