Toponogov, V.A. (1995). "On conditions for existence of umbilical points on a convex surface". Siberian Mathematical Journal. 36 (4): 780–784. doi:10.1007/BF02107335. S2CID122221022.
Fontenele, F.; Xavier, F. (2019). "Finding umbilics on open convex surfaces". Rev. Mat. Iberoam. 35 (7): 2035–2052. doi:10.4171/rmi/1109. S2CID199122809.
Toponogov, V.A. (1995). "On conditions for existence of umbilical points on a convex surface". Siberian Mathematical Journal. 36 (4): 780–784. doi:10.1007/BF02107335. S2CID122221022.
Fontenele, F.; Xavier, F. (2019). "Finding umbilics on open convex surfaces". Rev. Mat. Iberoam. 35 (7): 2035–2052. doi:10.4171/rmi/1109. S2CID199122809.