1. E. Cartan, On manifolds with projective connections, Bull. Soc. Math. France 52, 205–241 (1924).
2. V. Kagan, Subprojective Spaces (Fizmatgiz, Moscow, 1961). [in Russian]
3. G. Vranceanu, Proprietati globale ale spatiilor bui Riemann cu conexiune abina constanta, Stud. si cerc. mat. Acad. RPR 14, 7–22 (1963).
4. H. Weyl, Zur infinitesimal geometrie einordnung der projectiven und der konformen auffassung, Gottinger Nachtr., 99–112 (1921).
5. K. Yano, Concircular geometry, I-IV, Info Proc. Imp. Acad. Tokyo 16, 195–200; 354–360; 442–448; 505–511 (1940).