1. A. R. Forsyth, Geometry of Four Dimension (Cambridge Univ. Press, Cambridge, 1930), Vol. 1.
2. G. Ganchev and V. Milousheva, ‘‘Invariants and Bonnet-type theorem for surfaces in $$\mathbb{R}^{4}$$,’’ Centr. Eur. J. Math. 8, 993–1008 (2010).
3. R. Garcia and J. Sotomayor, ‘‘Structurally stable configurations of lines of mean curvature and umbilic points on surfaces immersed in $${\mathbb{R}}^{3}$$,’’ Publ. Mat. 45, 431–466 (2001).
4. R. Garcia and J. Sotomayor, ‘‘Historical comments on Monge’s ellipsoid and the confgurations of lines of curvature on surfaces,’’ Antiq. Math. 10, 169–182 (2016).
5. R. Garcia and J. Sotomayor, ‘‘Differential equations of classical geometry, a qualitative theory,’’ in Publicações Matemáticas, Proceedings of the 27th Brazilian Colloquium on Mathematics (IMPA, 2009).