Guilfoyle, B.; Klingenberg, W. (2004). "Generalised surfaces in ". Math. Proc. R. Ir. Acad. 104A (2): 199–209. doi:10.1353/mpr.2004.0013. S2CID118128548.
Guilfoyle, B.; Klingenberg, W. (2004). "Generalised surfaces in ". Math. Proc. R. Ir. Acad. 104A (2): 199–209. doi:10.1353/mpr.2004.0013. S2CID118128548.
Sotomayor, J.; Mello, L. F. (1999). "A note on some developments on Carathéodory conjecture on umbilic points". Exposition Math. 17 (1): 49–58. ISSN0723-0869.
Gutierrez, C.; Sotomayor, J. (1998). "Lines of curvature, umbilic points and Carathéodory conjecture". Resen. Inst. Mat. Estat. Univ. São Paulo. 3 (3): 291–322. ISSN0104-3854.