1. BUEKENHOUT, F. Ensembles quadratiques des espaces projectifs. Math. Z. 110 (1969), 306–318.
2. - Characterizations of semi-quadrics: a survey. To appear in Atti Colloq.Teo.Combin. Roma.
3. - On weakly perspective subsets of desarguesian projective lines. To appear in Abh. Math.Sem.Hamburg.
4. - The structure of semi-quadrics in projective spaces. (To appear).
5. BUEKENHOUT, F. and LEFEVRE, C. Generalized quadrangles in projective spaces. To appear in Archiv.der Math.