1. Heil E.: Relative and affine normals. Results Math. 13, 240–254 (1988)
2. Manhart F.: Relativgeometrische Kennzeichnungen Euklidischer Hypersphären. Geom. Dedicata 29, 193–207 (1989)
3. Schirokow, P.A., Schirokow, A.P.: Affine Differentialgeometrie. B. G. Teubner Verlagsgesellschaft, Leipzig (1962)
4. Stamatakis, S., Kaffas, I., Papadopoulou, I.-I.: Characterizations of ruled surfaces in $${\mathbb{R}^{3}}$$ R 3 and of hyperquadrics in $${\mathbb{R}^{n+1}}$$ R n + 1 via relative geometric invariants. J. Geom. Graph. 18, 217–223 (2014)